feat(planning): start_planning hands back the active plan that already covers the work (#4079)

Step 4 of milestone 415 "An existing plan is found before a new one is made".
A session that could not see an existing plan made a second one beside it.
start_planning and create_milestone now ask first: an ACTIVE milestone in the
project with the same title, or one that reads as the same plan (title, design
and steps against milestone embeddings), is returned with its progress and a
pointer to create_records(milestone_id=...). Nothing is created; force=true
bypasses.

- dedup.find_matching_plan / plan_gate / plan_match_response; access-checked
  before either arm (rule 78), fail-open like the other gates.
- Done milestones never block; the semantic arm needs 200+ chars of candidate.
- kb_plan_match_threshold (default 0.90) is a setting, in the Settings view,
  and pinned against the Python default by test_settings_defaults_agree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/services/dedup.py

@@ -33,6 +33,7 @@ from scribe.models.embedding import NoteEmbedding
 from scribe.models.note import Note
 from scribe.models.rulebook import Rule
 from scribe.models.base import iso
+from scribe.services.access import can_read_project
 from scribe.services import embeddings as embeddings_svc
 # Imported rather than redeclared: no service imports this module (the create
 # gate is called from the routes/tools layer), so there is no cycle to dodge,
@@ -720,3 +721,175 @@ async def find_duplicate_rule(
     except Exception:
         logger.debug("dedup rule title check skipped — query failed", exc_info=True)
     return None
+
+
+# --- the plan gate (milestone 415) -------------------------------------------
+# A session asked "what work is open?" that cannot see an existing plan makes a
+# second one: a new milestone beside the one that already covers the work, or
+# loose tasks beside it. Each copy then collects its own steps, and neither
+# shows the whole. This gate asks the question before start_planning (or
+# create_milestone) writes: is there an ACTIVE plan in this project for this?
+#
+# Active only: a done milestone is history, and planning the next round of
+# the same area is legitimate work rather than a copy of it.
+#
+# Project-scoped, not owner-scoped like the note gate. The note gate refuses to
+# point at someone else's record because they may not be able to edit it; a
+# plan is different. Creating one needs write on the project, and write on the
+# project is exactly what adding steps to its existing plan needs, so a caller
+# who reaches this gate can act on whatever it returns.
+#
+# Its own threshold, as a setting (rule 25). The note gate's 0.90 is where it
+# starts, because it asks the same question ("the same thing, reworded") with
+# the same embedder. Plan documents are shaped differently, though: a milestone
+# is embedded as title, description and plan (embeddings.milestone_document),
+# while the candidate usually has no description and carries its steps
+# instead. That difference has not been measured, and a gate that blocks on
+# noise teaches sessions to pass force=true every time, which is worse than no
+# gate. So the default is conservative and the operator can lower it.
+PLAN_MATCH_THRESHOLD_KEY = "kb_plan_match_threshold"
+PLAN_MATCH_DEFAULT_THRESHOLD = 0.90
+
+
+async def get_plan_match_threshold(user_id: int) -> float:
+    """The user's plan-gate similarity floor, clamped to [0, 1]."""
+    from scribe.services.settings import get_setting
+
+    try:
+        value = float(await get_setting(
+            user_id, PLAN_MATCH_THRESHOLD_KEY, str(PLAN_MATCH_DEFAULT_THRESHOLD)
+        ))
+    except (TypeError, ValueError):
+        value = PLAN_MATCH_DEFAULT_THRESHOLD
+    return min(1.0, max(0.0, value))
+
+
+def plan_candidate_text(
+    description: str | None = None,
+    body: str | None = None,
+    step_texts: list[str] | None = None,
+) -> str:
+    """What a plan that doesn't exist yet says about itself, for the gate.
+
+    The steps belong in it: a plan passed with steps and no design is still
+    recognisable by them, and what its steps say is most of what makes two
+    plans the same plan.
+    """
+    parts = [(description or "").strip(), (body or "").strip()]
+    parts += [t.strip() for t in (step_texts or [])]
+    return "\n\n".join(p for p in parts if p)
+
+
+async def find_matching_plan(
+    user_id: int,
+    project_id: int,
+    title: str,
+    text: str = "",
+) -> DuplicateMatch | None:
+    """An ACTIVE milestone in the project that already is this plan, or None.
+
+    Normalized-title match first, then semantic when `text` (from
+    plan_candidate_text) is long enough to mean something, the same floor the
+    note gate uses and for the same reason: a title-only embedding sits in a
+    tight neighbourhood and false-positives. Never raises; a failed check lets
+    the plan through, because a create must not depend on a recall aid.
+    """
+    from scribe.models.milestone import Milestone
+
+    if not project_id:
+        return None
+    # Rule 78, before either arm: a match names a milestone, and a caller who
+    # cannot read the project must not learn its plans by guessing titles.
+    try:
+        if not await can_read_project(user_id, project_id):
+            return None
+    except Exception:
+        logger.debug("plan gate access check failed — letting the plan through", exc_info=True)
+        return None
+    norm = " ".join((title or "").split()).lower()
+    if norm:
+        try:
+            async with async_session() as session:
+                existing = (await session.execute(
+                    select(Milestone).where(
+                        Milestone.project_id == project_id,
+                        Milestone.deleted_at.is_(None),
+                        Milestone.status == "active",
+                        func.lower(func.trim(Milestone.title)) == norm,
+                    ).limit(1)
+                )).scalars().first()
+            if existing is not None:
+                return DuplicateMatch(existing.id, existing.title, 1.0, "title")
+        except Exception:
+            logger.debug("plan gate title check skipped — query failed", exc_info=True)
+            return None
+
+    if len((text or "").strip()) < _MIN_BODY_FOR_SEMANTIC:
+        return None
+    doc_title, doc_body = embeddings_svc.milestone_document(title, None, text)
+    query = "\n\n".join(p for p in (doc_title, doc_body) if p)
+    try:
+        hits = await embeddings_svc.semantic_search_milestones(
+            user_id, query, project_id=project_id, status="active", limit=1,
+            threshold=await get_plan_match_threshold(user_id),
+        )
+    except Exception:
+        logger.debug("plan gate semantic check skipped", exc_info=True)
+        return None
+    if hits:
+        score, milestone = hits[0]
+        return DuplicateMatch(milestone.id, milestone.title, round(score, 3), "semantic")
+    return None
+
+
+def plan_match_response(dup: DuplicateMatch, progress: dict | None = None) -> dict:
+    """The payload start_planning / create_milestone return instead of a second
+    plan: the existing one, how far along it is, and how to add to it."""
+    progress = progress or {}
+    total, completed = progress.get("total", 0), progress.get("completed", 0)
+    how = "has the same title" if dup.reason == "title" else "reads as the same plan"
+    return {
+        "duplicate": True,
+        "existing_id": dup.id,
+        "existing_title": dup.title,
+        "existing_milestone": {
+            "id": dup.id,
+            "title": dup.title,
+            "description": progress.get("description") or "",
+            "total": total,
+            "completed": completed,
+        },
+        "similarity": dup.similarity,
+        "match": dup.reason,
+        "message": (
+            f'An active plan in this project {how}: milestone {dup.id} '
+            f'"{dup.title}" ({completed} of {total} steps done). Nothing was '
+            f"created. Add your steps to it with create_records(milestone_id="
+            f"{dup.id}, ...), and revise its design with update_milestone if the "
+            f"scope has grown. Read it first with get_milestone({dup.id}). If this "
+            f"really is a separate plan, retry with force=true."
+        ),
+    }
+
+
+async def plan_gate(
+    user_id: int,
+    project_id: int,
+    title: str,
+    text: str = "",
+) -> dict | None:
+    """find_matching_plan, answered: the plan_match_response to return in
+    place of a new plan, or None to go ahead and create it."""
+    from scribe.services import milestones as milestones_svc
+
+    dup = await find_matching_plan(user_id, project_id, title, text)
+    if dup is None:
+        return None
+    try:
+        rows = await milestones_svc.get_project_milestone_summary(user_id, project_id)
+        progress = next((r for r in rows if r.get("id") == dup.id), None)
+    except Exception:
+        # The match stands without its progress; losing the count must not
+        # turn a found plan into a second one.
+        progress = None
+    return plan_match_response(dup, progress)