feat(lessons): soft links — a lesson and a rule arriving together in distinct situations are proposed as a link (milestone 440 step 3, #4637)

When one hook response puts a lesson and a rule in front of the reader, the
pair is recorded as evidence on a SUGGESTED lesson_rule_links row; once the
pair has arrived together in PROPOSE_SITUATIONS (3) distinct situations, the
next co-arrival carries one line asking the reader to judge it with
judge_lesson_link. Nothing about surfacing changes: a suggested link carries
no rule anywhere (that is #4633, confirmed links only).

- lesson_rules: co_surfaced (fail-open; only pairs the reader could confirm —
  a lesson they may write, a rule they own; judged pairs gather nothing; one
  proposal per response; PROPOSE_COOLDOWN 6h between asks), plus the pure
  counting rules: situation_key, add_evidence, proposal_due, evidence_summary.
  A situation is the prompt on /retrieve (word tokens, sorted and
  de-duplicated, so trivial rewordings count once) and the FILE on
  /prior-art (every edit to one file is one situation).
- rules_for_lessons shows a suggested link's evidence counts.
- plugin_context: build_autoinject_hint returns lesson_ids,
  build_prompt_rule_hint returns shown_rule_ids, build_write_path_hint records
  its own pair; routes/plugin /retrieve records the prompt pair. Shown lines,
  repeats included: relevance makes a co-arrival, not the session ledger.
- Evidence lives in the existing evidence column — no migration, and backup
  already carries it.
- Tests: counting rules (unit), the recorder against Postgres (bar, repeat,
  judged pairs, ownership, cooldown, evidence kept on confirm); conftest stubs
  co_surfaced for unit tests.

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

tests/conftest.py

@@ -154,7 +154,10 @@ def _no_lesson_rule_links(request):
 
     The candidate search a new lesson runs (#4631) is stubbed to "nothing
     resembles it" for the same reason: it embeds, and the model load would
-    otherwise ride along with every unit test that creates a lesson.
+    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.
 
     Skipped for integration tests, which exercise the real links against
     Postgres (tests/test_integration_lesson_rule_links.py).
@@ -164,7 +167,8 @@ def _no_lesson_rule_links(request):
         return
     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.rule_candidates", AsyncMock(return_value=[])), \
+         patch("scribe.services.lesson_rules.co_surfaced", AsyncMock(return_value="")):
         yield