feat(lessons): judged when written — a new lesson is offered its rules, and "no rule fits" is an answer (milestone 440 step 2, #4631)

"Which rule is this lesson an instance of?" now has three recorded answers:
a rule named (a confirmed link, #4630), no rule fits (new), or unjudged.

- Model + migration 0112: lesson_no_rule (lesson_id PK, CASCADE from the
  note; why; judged_at). A table rather than a key in notes.data, because
  that mirror is re-composed from the body on every edit and would erase it.
- Service (lesson_rules): set_no_rule rejects any confirmed link with the
  reason; a confirmation (set_lesson_rules or judge_link) deletes the answer;
  require_one_answer refuses both answers in one call before any write;
  judgments_for_lessons + attach_lesson_rules add rule_judgment (and no_rule)
  to every lesson payload; list_unjudged lists the open ones; rule_candidates
  searches rules with the lesson's claim + trigger at the explicit-search bar,
  None when the search could not run.
- MCP: create_lesson/update_lesson take no_rule; an unanswered create returns
  rule_candidates, rule_judgment and a rule_hint; list_lessons(unjudged=true).
- REST: the same on POST/PATCH /api/lessons and GET ?unjudged=1; create
  returns rule_candidates.
- Backup v19: a lesson_no_rule section, export (full and per-user) and import.
- Guidance: create_lesson docstring, writing-records.md in using-scribe (owner,
  pinned in test_guidance_ownership), create_rule docstring on linking the
  lessons a new rule governs. Plugin version minted.
- Tests: door units, integration for the three states, the rejection reason,
  scoping, cascade; backup registries.

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

tests/conftest.py

@@ -152,6 +152,10 @@ def _no_lesson_rule_links(request):
     Both decorations are fail-open, so the cost would be a slow failed connect
     per test rather than a failure, which is worse: it would never be noticed.
 
+    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.
+
     Skipped for integration tests, which exercise the real links against
     Postgres (tests/test_integration_lesson_rule_links.py).
     """
@@ -159,7 +163,8 @@ def _no_lesson_rule_links(request):
         yield
         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.attach_rule_lessons", AsyncMock()), \
+         patch("scribe.services.lesson_rules.rule_candidates", AsyncMock(return_value=[])):
         yield