feat(moments): mounted rules arrive when their moment happens, through every door (milestone 458 step 4a, #4922)

A rule mounted on a moment now reaches the session when an act reaches
that moment, with no semantic match involved:

- run_moment_arm on the pipeline: a lookup, not a ranked search. Each
  line names the moment and the act that reached it ("at work.deliver,
  reached by `git push`"), so a misfire is visible where it lands and
  can be unmapped in-session. A repeat is cited, not quoted; fresh
  rules are recorded surfaced under source moment_rule with the moment
  in detail. No retrieval_logs row, as for the other lookups, so no
  latency is persisted for this arm.
- rule_scope: a rule's home clause, moved out of semantic_search_rules
  so the moment lookup scopes by the same one.
- rulebooks.rules_on_moments / mounted_moments.
- The plugin door: a catch-all PreToolUse hook (scribe_moment.sh). It
  keeps /moment-tools' answer on disk for five minutes, so a call to a
  tool that cannot reach a mounted rule sends nothing, and an install
  that has mounted nothing sends one request per window. It shares the
  rules ledger with the other arms and fails open silently.
- The MCP door: Scribe's own tools named by the shipped mappings carry
  moment_rules in their response, so a client without the plugin gets
  them too. The hook skips those tools. A guard pins the attach on
  every one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/services/rulebooks.py

@@ -1160,6 +1160,70 @@ async def list_rule_moments(rule_ids: list[int]) -> dict[int, list[str]]:
     return out
 
 
+async def rules_on_moments(
+    user_id: int, moments: list[str], project_id: int | None = None,
+) -> list[tuple[Rule, str]]:
+    """The rules mounted on any of these moments that this caller may be shown.
+
+    The delivery read (milestone 458 step 4), and a LOOKUP: no score, no bar.
+    Scoped by the same home clause the semantic search uses (rule_scope), so
+    a mount never delivers another project's rule. Each rule appears once,
+    under the first of `moments` it is mounted on — the caller passes them in
+    catalog order, so that is the most specific moment the work reached.
+    """
+    from scribe.models.rulebook import rule_moments as rule_moments_t
+    from scribe.services.rule_scope import joined_to_homes, rule_home
+
+    if not moments:
+        return []
+    home = await rule_home(user_id, project_id)
+    async with async_session() as session:
+        rows = (await session.execute(
+            joined_to_homes(
+                select(Rule, rule_moments_t.c.moment)
+                .select_from(rule_moments_t)
+                .join(Rule, Rule.id == rule_moments_t.c.rule_id)
+            )
+            .where(
+                rule_moments_t.c.moment.in_(list(moments)),
+                Rule.deleted_at.is_(None),
+                home,
+            )
+            .order_by(Rule.id)
+        )).all()
+    rank = {m: i for i, m in enumerate(moments)}
+    first: dict[int, tuple[Rule, str]] = {}
+    for rule, moment in rows:
+        held = first.get(rule.id)
+        if held is None or rank[moment] < rank[held[1]]:
+            first[rule.id] = (rule, moment)
+    return sorted(first.values(), key=lambda pair: (rank[pair[1]], pair[0].id))
+
+
+async def mounted_moments(user_id: int) -> set[str]:
+    """Every moment at least one of this caller's live rules is mounted on.
+
+    In ANY home (rule_scope's `everywhere`): this answers "can a call reach
+    anything at all", which is what lets the plugin stay off the wire for a
+    tool whose moments carry nothing. A superset is the safe direction — the
+    delivery read still scopes by project.
+    """
+    from scribe.models.rulebook import rule_moments as rule_moments_t
+    from scribe.services.rule_scope import joined_to_homes, rule_home
+
+    home = await rule_home(user_id, everywhere=True)
+    async with async_session() as session:
+        rows = (await session.execute(
+            joined_to_homes(
+                select(rule_moments_t.c.moment).distinct()
+                .select_from(rule_moments_t)
+                .join(Rule, Rule.id == rule_moments_t.c.rule_id)
+            )
+            .where(Rule.deleted_at.is_(None), home)
+        )).scalars().all()
+    return set(rows)
+
+
 async def list_rule_systems(rule_ids: list[int]) -> dict[int, list[dict]]:
     """The canon tags for a batch of rules, keyed by rule id.