feat(rules): a rule surfaces through the lessons confirmed as its instances (milestone 440 step 4, #4633)

On all three rule arms (prompt_rule, pre_tool_rule, write_path_rule), after
the direct match, a lesson matching the moment at the notes menu's own bar
brings the rule(s) it is CONFIRMED to be an instance of, rendered in rule
voice with "Reached through lesson #N “…”, a recorded instance of it."

- Confirmed links only (lesson_rules.confirmed_lessons /
  confirmed_rules_in_scope). A suggested link carrying its rule would
  manufacture the co-arrival #4637 counts and prove itself.
- Scope kept: a lesson surfaces everywhere, but the rule it brings must be
  global or this project's own — milestone 414's boundary, not reopened
  through a side door.
- Suppression is by RULE: anything the direct band named or the session
  ledger holds is skipped, whichever lesson reached it.
- Its own slot (VIA_LESSON_LIMIT = 1), not a rule slot — a stated default,
  since the plan's "decide by measurement" has nothing to measure until
  links are confirmed (#4632). Its own source, rule_via_lesson: registered,
  RANKED for pull-through, logged whenever it searches.
- Nothing searched at all while no lesson carries a confirmed link, which is
  every install until one is judged.
- build_prompt_rule_hint / build_tool_rule_hint are now thin wrappers over
  the direct arms (_prompt_rule_hint / _tool_rule_hint); the arm leaves its
  query in `_via_query` only when it ran, so a disabled arm or a blank prompt
  brings no rule in either, and the key never leaves the server.
- Via-lesson rules are not fed to the #4637 co-surfacing recorder: only
  direct matches are evidence.

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

src/scribe/services/retrieval_registry.py

@@ -137,6 +137,12 @@ POINTS: dict[str, Point] = dict([
        "the one line reserved for a preference at the prompt boundary"),
     _p("reuse_slot", UNBIDDEN, "the one line reserved for a reusable snippet"),
     _p("lesson_slot", UNBIDDEN, "the one line reserved for a lesson"),
+    _p("rule_via_lesson", UNBIDDEN,
+       "a rule reached through a lesson confirmed as an instance of it",
+       expects_traffic=False,
+       quiet_because="searches only once some lesson has a confirmed link to "
+                     "a rule; an install where none has been judged is "
+                     "correctly silent here"),
     # `fixed_query`: COMPLETION_QUERY is a module constant in
     # services/reply_preferences.py, so this arm's top score is the same number
     # on every call — measured at 0.791 across 45 consecutive calls, with p10,