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/retrieval_registry.py

@@ -163,6 +163,15 @@ POINTS: dict[str, Point] = dict([
        "the fixed question asked when a task finishes: how should this report read",
        fixed_query=True),
 
+    # The moment arm (milestone 458). A LOOKUP, not a ranker: a rule mounted on
+    # a moment arrives when that moment happens, with no score, so it writes no
+    # retrieval_logs row and no score-shaped warning can apply. Its rows are in
+    # `rule_usage_events`, each carrying the moment in `detail`.
+    _p("moment_rule", UNBIDDEN,
+       "rules mounted on a moment of work, delivered when that moment happens",
+       expects_traffic=False,
+       quiet_because="speaks only once some rule is mounted on a moment; an "
+                     "install where none is mounted is correctly silent here"),
     # The rulings arm (milestone 444). A LOOKUP, not a ranker: a path falls
     # under a System's patterns or it does not, so nothing here writes to
     # retrieval_logs and no score-shaped warning can apply. Its rows are in