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>

tests/conftest.py

@@ -157,7 +157,10 @@ def _no_lesson_rule_links(request):
     otherwise ride along with every unit test that creates a lesson. The
     co-surfacing recorder (#4637) is stubbed to "no proposal": every hook
     builder test that renders a lesson beside a rule would otherwise reach
-    for the database to count the pair.
+    for the database to count the pair. And the via-lesson rule step (#4633)
+    is stubbed to "no lesson brought a rule": all three rule arms run it, and
+    its first act is a database read. tests/test_rule_via_lesson.py binds the
+    real function at import time, before this patch runs.
 
     Skipped for integration tests, which exercise the real links against
     Postgres (tests/test_integration_lesson_rule_links.py).
@@ -168,7 +171,9 @@ def _no_lesson_rule_links(request):
     with patch("scribe.services.lesson_rules.attach_lesson_rules", AsyncMock()), \
          patch("scribe.services.lesson_rules.attach_rule_lessons", AsyncMock()), \
          patch("scribe.services.lesson_rules.rule_candidates", AsyncMock(return_value=[])), \
-         patch("scribe.services.lesson_rules.co_surfaced", AsyncMock(return_value="")):
+         patch("scribe.services.lesson_rules.co_surfaced", AsyncMock(return_value="")), \
+         patch("scribe.services.plugin_context._rules_via_lessons",
+               AsyncMock(return_value=([], []))):
         yield