feat(mcp): Phase 5 — write-time near-duplicate gate (update-over-create)

#755 Phase 5. create_note / create_task now BLOCK a near-duplicate instead of
silently inserting: they return {"duplicate": true, "existing_id", message}
pointing at the record to UPDATE. Fights store bloat and stale competing copies
that semantic search (RAG) would otherwise resurface for reconciliation. A
force=true override creates anyway for genuinely-distinct records.

- services/dedup.py: find_duplicate_note — two signals, scoped to owner + same
  project + same kind: (1) normalized-title exact match (cheap, always); (2)
  semantic cosine ≥ 0.90 but ONLY when body ≥ 200 chars (short/title-only
  embeddings false-positive — the pre-pivot lesson). Project-less (orphan)
  records compare only to other orphans on BOTH signals (orphan_only on the
  semantic call) — they're not matched across every project.
- Gate wired into the MCP create_note/create_task tools (the LLM write path)
  with force override; _INSTRUCTIONS documents the duplicate response + force.
- Opt-in by design: the service helper is only called from the interactive
  create tools. Internal/programmatic creates (recurrence spawn, imports) go
  straight through services.create_note and are NOT gated — a recurring task
  spawning its next same-titled instance must not be blocked.
- Scope v1: MCP tools only. REST/web (human CRUD, needs a UI affordance) and
  create_rule (not a RAG surface; _INSTRUCTIONS already steer it) are follow-ups.
- tests: dedup service (title/semantic/body-gate/type-filter) + tool gate
  (blocks, force bypasses) for notes and tasks.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/scribe/services/dedup.py

@@ -0,0 +1,124 @@
+"""Write-time near-duplicate detection — the update-over-create gate.
+
+Goal: stop a second near-identical row from being created when an existing one
+should be UPDATED instead. Duplicates bloat the store and, worse, get surfaced
+by semantic search (RAG) later as competing/stale copies that then have to be
+reconciled. This is the enforcement half of the instruction-level "prefer
+updating over creating" reflex.
+
+OPT-IN by design: the interactive create paths (MCP create tools + REST create
+routes) run this gate; internal/programmatic creates do NOT (e.g. a recurring
+task spawning its next instance, or a bulk import — those legitimately repeat a
+title and must not be blocked). Callers that want the gate call find_duplicate_*
+themselves and act on a hit; nothing here mutates.
+
+Two signals, both scoped to the same owner + project + kind:
+  1. Normalized-title exact match — cheap, always checked.
+  2. Semantic similarity (cosine ≥ _SEMANTIC_THRESHOLD) — only when the incoming
+     body is substantial. Short/title-only embeddings sit in a tight neighborhood
+     and false-positive (the pre-pivot lesson: "Lore: Shell 0" vs
+     "Lore: Reinitialization 0" matched at 0.91 with no body), so we gate it on
+     a minimum body length.
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from sqlalchemy import func, select
+
+from scribe.models import async_session
+from scribe.models.note import Note
+from scribe.services import embeddings as embeddings_svc
+
+# Run the semantic check only when the incoming body has at least this many
+# characters — below it, embeddings are dominated by the title and false-positive.
+_MIN_BODY_FOR_SEMANTIC = 200
+# Cosine threshold for "this is the same thing, reworded." Deliberately high to
+# keep false positives rare (a hard block with a force-override is unforgiving of
+# noise). Matches the 0.90 the pre-pivot dedup settled on.
+_SEMANTIC_THRESHOLD = 0.90
+
+
+@dataclass
+class DuplicateMatch:
+    """An existing record judged a near-duplicate of an incoming create."""
+    id: int
+    title: str
+    similarity: float  # 1.0 for an exact normalized-title match
+    reason: str        # "title" | "semantic"
+
+
+def duplicate_response(dup: "DuplicateMatch", kind: str) -> dict:
+    """Standard 'blocked — update instead' payload returned by a create tool
+    when the gate finds a near-duplicate. `kind` is 'note' or 'task' (drives the
+    update_<kind> hint)."""
+    return {
+        "duplicate": True,
+        "existing_id": dup.id,
+        "existing_title": dup.title,
+        "similarity": dup.similarity,
+        "match": dup.reason,
+        "message": (
+            f'A {dup.reason}-similar {kind} already exists (id {dup.id}: '
+            f'"{dup.title}"). Prefer UPDATING it (update_{kind}) over creating a '
+            f"near-duplicate. If this really is a distinct {kind}, retry with "
+            f"force=true."
+        ),
+    }
+
+
+async def find_duplicate_note(
+    user_id: int,
+    title: str,
+    body: str = "",
+    project_id: int | None = None,
+    is_task: bool | None = None,
+    note_type: str = "note",
+) -> DuplicateMatch | None:
+    """Best near-duplicate of (title, body) within the same owner + project +
+    kind, or None. Title match first (cheap, exact), then semantic when the body
+    is long enough to be meaningful. Never raises — embedder failure degrades to
+    title-only (callers should still be able to create)."""
+    norm = " ".join((title or "").split()).lower()
+
+    # --- Signal 1: normalized-title exact match (same scope) ---
+    if norm:
+        async with async_session() as session:
+            stmt = select(Note).where(
+                Note.user_id == user_id,
+                Note.deleted_at.is_(None),
+                Note.note_type == note_type,
+                func.lower(func.trim(Note.title)) == norm,
+            )
+            if project_id is not None:
+                stmt = stmt.where(Note.project_id == project_id)
+            else:
+                stmt = stmt.where(Note.project_id.is_(None))
+            if is_task is True:
+                stmt = stmt.where(Note.status.isnot(None))
+            elif is_task is False:
+                stmt = stmt.where(Note.status.is_(None))
+            existing = (await session.execute(stmt.limit(1))).scalars().first()
+            if existing is not None:
+                return DuplicateMatch(existing.id, existing.title, 1.0, "title")
+
+    # --- Signal 2: semantic similarity (only with a substantial body) ---
+    if body and len(body.strip()) >= _MIN_BODY_FOR_SEMANTIC:
+        query = f"{title}\n{body}".strip()
+        # Scope the semantic check the same way as the title check: a record in
+        # project P compares only to P; a project-less (orphan) record compares
+        # only to other orphans (orphan_only), NOT across every project — without
+        # this, semantic_search_notes applies no project filter when project_id
+        # is None and would match an orphan note against any project's notes.
+        hits = await embeddings_svc.semantic_search_notes(
+            user_id, query, project_id=project_id, is_task=is_task,
+            orphan_only=(project_id is None),
+            limit=3, threshold=_SEMANTIC_THRESHOLD,
+        )
+        for score, note in hits:
+            # semantic_search_notes doesn't filter note_type — enforce it here so
+            # a note doesn't shadow a task of the same wording, etc.
+            if note.note_type == note_type:
+                return DuplicateMatch(note.id, note.title, round(score, 3), "semantic")
+
+    return None