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>

alembic/versions/0112_lesson_no_rule.py

@@ -0,0 +1,33 @@
+"""lesson_no_rule — "no rule fits" is a recorded answer (milestone 440 step 2,
+#4631)
+
+Revision ID: 0112
+Revises: 0111
+Create Date: 2026-10-01
+
+One row per lesson that was judged to fall under no rule, with the reason. A
+lesson with neither a confirmed link nor this row is UNJUDGED, which is what
+`list_lessons(unjudged=true)` lists. CASCADE from the lesson. No backfill: no
+lesson has been judged yet, and writing "no rule fits" for one nobody looked
+at would assert a judgment nobody made.
+"""
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0112"
+down_revision = "0111"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "lesson_no_rule",
+        sa.Column("lesson_id", sa.Integer(), sa.ForeignKey("notes.id", ondelete="CASCADE"), primary_key=True),
+        sa.Column("why", sa.Text(), nullable=False),
+        sa.Column("judged_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.text("now()")),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("lesson_no_rule")