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/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