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/rule_usage.py

@@ -101,6 +101,11 @@ logger = logging.getLogger(__name__)
 # Add a source here only when a ranker picked it.
 RANKED_SOURCES = (
     "write_path_rule", "pre_tool_rule", "prompt_rule",
+    # A rule mounted on a moment (milestone 458). Not a ranker's pick, but
+    # not bulk either: somebody decided this rule applies at this moment, and
+    # that is exactly the claim a pull can confirm or refute. Its
+    # pull-through is the evidence for whether a mount earns its line.
+    "moment_rule",
     # A reserved slot is a ranker's choice twice over — it ran a query AND
     # decided a kind was worth guaranteeing a place. Left out, its line would
     # be counted as bulk delivery and drop out of the denominator, so the one
@@ -154,7 +159,8 @@ def _schedule(rows: list[dict]) -> None:
 
 
 def record_rule_surfaced(
-    *, user_id: int | None, rule_ids: list[int] | set[int], source: str
+    *, user_id: int | None, rule_ids: list[int] | set[int], source: str,
+    detail: dict[int, str] | None = None,
 ) -> None:
     """Fire-and-forget: record that these rules were shown to the agent.
 
@@ -173,6 +179,10 @@ def record_rule_surfaced(
     from the other end — everything in a preload IS shown. `source` is what
     separates the two afterwards (see `RANKED_SOURCES`); this function does not
     care which kind it is recording.
+
+    `detail` maps a rule id to what to record beside its row — the moment a
+    mounted rule arrived at (milestone 458), so a mount's pull-through can be
+    read per moment rather than only per arm.
     """
     try:
         rows = [
@@ -181,6 +191,7 @@ def record_rule_surfaced(
                 "rule_id": int(rid),
                 "event": SURFACED,
                 "source": source,
+                **({"detail": detail[rid]} if detail and rid in detail else {}),
             }
             for rid in rule_ids
         ]