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_pipeline.py

@@ -662,3 +662,100 @@ async def run_rule_arm(
     except Exception:  # noqa: BLE001 - a recall aid never breaks its act
         logger.debug("%s arm failed", arm.source, exc_info=True)
         return RuleResult()
+
+
+# ── The moment arm (milestone 458) ───────────────────────────────────────
+#
+# A LOOKUP beside the ranked arms, not one of them. A rule mounted on a moment
+# arrives because somebody said it belongs there, not because the work's words
+# resemble it — which is the whole point: a rule about WHEN work is finished
+# never scores against what is said while finishing it. So there is no score,
+# no floor and no band, and it writes no retrieval_logs row (the rulings arm's
+# precedent, #4769): every score-shaped warning would misread a lookup. What it
+# records is the surfacing, with the moment beside each row, so a mount's
+# pull-through can be read per moment.
+#
+# It shares everything else with the ranked arms: the line renderer, the
+# session ledger (a repeat renders as a citation, never hidden — #3750) and
+# the fresh-only surfacing rows (#3752).
+
+MOMENT_RULE_SOURCE = "moment_rule"
+
+# A cap against a misconfiguration, not a ranking budget. Mounts are
+# deliberate, so the ordinary case is a handful; a rulebook that mounted
+# dozens of rules on one moment would otherwise bury the act it annotates.
+# The remedy for a moment that hits this is fewer mounts (unmount through
+# update_rule), which is why it is not a tuning dial.
+MOMENT_RULE_LIMIT = 8
+
+
+@dataclass(frozen=True)
+class MomentIO:
+    """The mounted-rule lookup and the recorder the moment arm reports to."""
+
+    lookup: Callable[..., Any]
+    """`rulebooks.rules_on_moments`, or a stand-in with its signature."""
+
+    record_rule_surfaced: Callable[..., Any]
+
+
+def _reached_by(hit: dict) -> str:
+    """How the line names what reached a moment: the action, as mapped."""
+    return f"`{hit.get('match') or hit.get('tool') or '?'}`"
+
+
+async def run_moment_arm(
+    reached: list[dict], moment: RuleMoment, *, io: MomentIO,
+    limit: int = MOMENT_RULE_LIMIT,
+) -> RuleResult:
+    """Deliver the rules mounted on the moments this act reached.
+
+    `reached` is `moment_actions.resolve`'s answer — each moment with the
+    action that reached it — and every line says both ("at work.deliver,
+    reached by `git push`"), so a moment that fired on the wrong act is
+    visible in the line itself and can be corrected in the session with
+    `unmap_action` (step 2's ruling). Fresh rules come first and carry their
+    trigger; a rule the session was already shown is cited, not repeated in
+    full. Fails open to an empty result.
+    """
+    try:
+        names = [hit["moment"] for hit in reached]
+        if not names:
+            return RuleResult()
+        pairs = await io.lookup(moment.user_id, names, moment.project_id or None)
+        if not pairs:
+            return RuleResult()
+        by_moment = {hit["moment"]: hit for hit in reached}
+        # Stable: catalog order within each half, fresh half first.
+        shown = sorted(pairs, key=lambda pair: pair[0].id in moment.exclude)[:limit]
+
+        lines = []
+        for rule, at in shown:
+            seen = rule.id in moment.exclude
+            lines.append(_rule_hint_line(
+                rule,
+                where=f"at {at}, reached by {_reached_by(by_moment.get(at, {}))}",
+                seen=seen, held=rule.id in moment.held,
+                # A repeat is cited rather than quoted: the session was told
+                # which moment brought it the first time.
+                compact=seen,
+            ))
+        fresh = [(rule, at) for rule, at in shown if rule.id not in moment.exclude]
+        rule_ids = [rule.id for rule, _at in fresh]
+        if rule_ids:
+            try:
+                io.record_rule_surfaced(
+                    user_id=moment.user_id, rule_ids=rule_ids,
+                    source=MOMENT_RULE_SOURCE,
+                    detail={rule.id: at for rule, at in fresh},
+                )
+            except Exception:  # noqa: BLE001 - observation never breaks the observed
+                _telemetry_failed(MOMENT_RULE_SOURCE)
+        return RuleResult(
+            lines=lines, rule_ids=rule_ids,
+            shown_rule_ids=[rule.id for rule, _at in shown],
+            shown=[(None, rule) for rule, _at in shown],
+        )
+    except Exception:  # noqa: BLE001 - a recall aid never breaks its act
+        logger.debug("%s arm failed", MOMENT_RULE_SOURCE, exc_info=True)
+        return RuleResult()