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>

tests/test_mcp_tool_rulebooks.py

@@ -94,6 +94,34 @@ async def test_create_rule_passes_required_fields():
     assert kwargs["statement"] == "Work directly on dev"
 
 
+@pytest.mark.asyncio
+async def test_create_rule_blocked_by_duplicate_gate():
+    from scribe.services.dedup import DuplicateMatch
+    dup = DuplicateMatch(id=47, title="dev is home", similarity=1.0, reason="title")
+    create_mock = AsyncMock()
+    with patch("scribe.mcp.tools.rulebooks.dedup_svc.find_duplicate_rule",
+               AsyncMock(return_value=dup)), \
+         patch("scribe.mcp.tools.rulebooks.rulebooks_svc.create_rule", create_mock):
+        from scribe.mcp.tools.rulebooks import create_rule
+        out = await create_rule(topic_id=10, title="dev is home", statement="x")
+    assert out["duplicate"] is True
+    assert out["existing_id"] == 47
+    assert "update_rule" in out["message"]
+    create_mock.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_create_rule_force_bypasses_duplicate_gate():
+    find_mock = AsyncMock()
+    with patch("scribe.mcp.tools.rulebooks.dedup_svc.find_duplicate_rule", find_mock), \
+         patch("scribe.mcp.tools.rulebooks.rulebooks_svc.create_rule",
+               AsyncMock(return_value=_fake_rule(id=5))):
+        from scribe.mcp.tools.rulebooks import create_rule
+        out = await create_rule(topic_id=10, title="dev is home", statement="x", force=True)
+    assert out["id"] == 5
+    find_mock.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_update_rule_only_sends_non_default_fields():
     rule = _fake_rule()