feat(mcp): extend dedup gate to create_rule / create_project_rule

Completes the Phase 5 follow-up: rules now get the same update-over-create
gate. Title-based only (rules aren't a semantic-retrieval/RAG surface), scoped
to the same topic (rulebook rule) or same project (project rule). force=true
overrides; fail-open like the note/task gate.

Deferred-item decisions (operator): REST/web gating SKIPPED (kept MCP-only —
humans rarely double-create and a hard block needs UI affordance); orphan scope
kept orphan↔orphan (no change). So this rule gate is the only remaining build.

- services/dedup.py: find_duplicate_rule(title, topic_id|project_id).
- create_rule + create_project_rule: force param + gate.
- tests: rule title match, scope-required guard, tool gate (block + force).

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

src/scribe/mcp/tools/rulebooks.py

@@ -10,6 +10,7 @@ spec.
 from __future__ import annotations
 
 from scribe.mcp._context import current_user_id
+from scribe.services import dedup as dedup_svc
 from scribe.services import rulebooks as rulebooks_svc
 from scribe.services import trash as trash_svc
 
@@ -257,6 +258,7 @@ async def get_rule(rule_id: int) -> dict:
 async def create_rule(
     topic_id: int, title: str, statement: str,
     why: str = "", how_to_apply: str = "", order_index: int = 0,
+    force: bool = False,
 ) -> dict:
     """Create a new rule in a rulebook (a SHARED rule — keep it general).
 
@@ -275,8 +277,15 @@ async def create_rule(
         why: Optional rationale — the reason the rule exists.
         how_to_apply: Optional operationalization — when / where it kicks in.
         order_index: Display order within the topic (default 0).
+        force: Bypass the near-duplicate gate. By default, a title-identical rule
+            already in this topic BLOCKS creation and returns its id so you update
+            it instead. Set true only for a genuinely distinct rule.
     """
     uid = current_user_id()
+    if not force:
+        dup = await dedup_svc.find_duplicate_rule(title, topic_id=topic_id)
+        if dup is not None:
+            return dedup_svc.duplicate_response(dup, "rule")
     rule = await rulebooks_svc.create_rule(
         topic_id=topic_id, user_id=uid,
         title=title, statement=statement,
@@ -288,6 +297,7 @@ async def create_rule(
 async def create_project_rule(
     project_id: int, statement: str, title: str = "",
     why: str = "", how_to_apply: str = "", order_index: int = 0,
+    force: bool = False,
 ) -> dict:
     """Create a rule scoped to a single project (no rulebook needed).
 
@@ -307,9 +317,16 @@ async def create_project_rule(
         why: Optional rationale — the reason the rule exists.
         how_to_apply: Optional operationalization — when / where it kicks in.
         order_index: Display order within the project's rule list (default 0).
+        force: Bypass the near-duplicate gate. By default, a title-identical rule
+            already on this project BLOCKS creation and returns its id so you
+            update it instead. Set true only for a genuinely distinct rule.
     """
     uid = current_user_id()
     derived_title = title.strip() or statement.strip().split(".")[0][:50]
+    if not force:
+        dup = await dedup_svc.find_duplicate_rule(derived_title, project_id=project_id)
+        if dup is not None:
+            return dedup_svc.duplicate_response(dup, "rule")
     rule = await rulebooks_svc.create_project_rule(
         project_id=project_id, user_id=uid,
         title=derived_title, statement=statement,

src/scribe/services/dedup.py

@@ -29,6 +29,7 @@ from sqlalchemy import func, select
 
 from scribe.models import async_session
 from scribe.models.note import Note
+from scribe.models.rulebook import Rule
 from scribe.services import embeddings as embeddings_svc
 
 logger = logging.getLogger(__name__)
@@ -131,3 +132,33 @@ async def find_duplicate_note(
                 return DuplicateMatch(note.id, note.title, round(score, 3), "semantic")
 
     return None
+
+
+async def find_duplicate_rule(
+    title: str,
+    topic_id: int | None = None,
+    project_id: int | None = None,
+) -> DuplicateMatch | None:
+    """Title-based near-duplicate of a rule, scoped to the same topic (a rulebook
+    rule) or the same project (a project rule). Rules aren't a semantic-retrieval
+    surface, so a normalized-title match is the right (and only) signal. Fail-open
+    like find_duplicate_note."""
+    norm = " ".join((title or "").split()).lower()
+    if not norm or (topic_id is None and project_id is None):
+        return None
+    try:
+        async with async_session() as session:
+            stmt = select(Rule).where(
+                Rule.deleted_at.is_(None),
+                func.lower(func.trim(Rule.title)) == norm,
+            )
+            if topic_id is not None:
+                stmt = stmt.where(Rule.topic_id == topic_id)
+            else:
+                stmt = stmt.where(Rule.project_id == project_id)
+            existing = (await session.execute(stmt.limit(1))).scalars().first()
+            if existing is not None:
+                return DuplicateMatch(existing.id, existing.title, 1.0, "title")
+    except Exception:
+        logger.debug("dedup rule title check skipped — query failed", exc_info=True)
+    return None