feat(lessons): convergence is named at the write — no-rule lessons that keep landing in one situation suggest a rule (milestone 440 step 5, #4634)

When a lesson is answered "no rule fits" (create_lesson / update_lesson on
both doors), the response looks for other no-rule lessons it resembles and,
once there are CONVERGENCE_LESSONS (3) of them, carries `convergence`: the
members, their incidents and projects, and a hint to draft the missing rule
with create_rule (operator approval as always) and point each lesson at it —
or to leave them as lessons when no single choice is right every time.

- convergence_group is the pure bar: distinct LESSONS count, incidents never
  stand in for them (one broad lesson cannot trigger it), and a group whose
  sources all point at one incident is one event written up several times.
- convergence_for searches lessons by the new one's claim + trigger
  (trigger_title) at CONVERGENCE_THRESHOLD 0.65 — above the menu's "worth
  showing", below the duplicate gate's "same record" — then keeps the ones
  with a lesson_no_rule answer. Fail-open. No sweep, no timer (#4183).
- Defaults stated as defaults (rules 32, 115).
- Tests: the bar (pure), the search with stubs, the door, and the no-rule
  filter against Postgres; conftest stubs convergence_for for unit tests.

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

tests/conftest.py

@@ -160,7 +160,9 @@ def _no_lesson_rule_links(request):
     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.
+    real function at import time, before this patch runs. The convergence
+    check a "no rule fits" answer runs (#4634) is stubbed to "no group" for
+    the same reason; tests/test_lesson_convergence.py binds the real one.
 
     Skipped for integration tests, which exercise the real links against
     Postgres (tests/test_integration_lesson_rule_links.py).
@@ -173,7 +175,8 @@ def _no_lesson_rule_links(request):
          patch("scribe.services.lesson_rules.rule_candidates", AsyncMock(return_value=[])), \
          patch("scribe.services.lesson_rules.co_surfaced", AsyncMock(return_value="")), \
          patch("scribe.services.plugin_context._rules_via_lessons",
-               AsyncMock(return_value=([], []))):
+               AsyncMock(return_value=([], []))), \
+         patch("scribe.services.lesson_rules.convergence_for", AsyncMock(return_value=None)):
         yield