feat(lessons): the lesson kind, and one join for every trigger title (#3729)

Milestone 385 step 2, implementing decision #4157 from the step-1 spike.

The kind: `note_type='lesson'`, a note findable by WHEN IT APPLIES rather
than by what it is about. The trigger lives in `notes.data.when_to_apply`,
mirrored into the title and the head of the body — the shape snippets
already use, and the reason nothing re-embeds: chunk_document is untouched,
so CHUNKER_VERSION does not move.

NO MIGRATION, and the step assumed there would be one. `note_type` carries
no CHECK — only `task_kind` does (0056, 0065). Migration 0036 added it as
plain Text with a server default and nothing has gated it since, so rule 36
has no whitelist to expand and #3128's failure mode (a value the database
refuses) cannot arise for this column. The vocabulary that actually decides
what a reader can reach is services.knowledge._FACETS, which since #3161 is
one table feeding the door's validation, the counts and both dialects of
the type filter — so the kind lands there in a single edit.

ONE JOIN, not a fourth copy. `{subject} — {trigger}` had three
implementations: rule_document, snippets.compose_title, and this step
needed another. #3207 records what that costs, so the join moves to
embeddings.trigger_title beside embedding_text and all three delegate.
Behaviour is unchanged for rules and snippets; the guard calls each through
its own public name, so a re-implementation fails it.

The #3163 bill is stated in the service docstring rather than left to be
inferred: versions, supersession, trash, the share ACL, tags, project and
System tagging, chunked embeddings and the duplicate gate are all
inherited; status/task_kind/milestone_id and recurrence are not, and
verify_with/expires_when are available but outside the kind's contract.
The status cell is the one that matters — `is_task` IS `status is not
None`, so a lesson that acquired one would become a task.

The integration guard asserts the WRITE rather than the constraint: it
holds whether or not note_type is ever gated, and goes red only if it is
gated without this value.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/models/note.py

@@ -76,8 +76,27 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
     recurrence_next_spawn_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )
-    # Note type — 'note' (default) or 'process' (a stored process). Task-ness is
-    # tracked by `status`, not here. (person/place/list entity types removed 2026-07.)
+    # WHAT KIND of record this is, on the note/entity axis. Task-ness is tracked
+    # by `status`, not here (person/place/list entity types removed 2026-07):
+    #   note    (default) — authored prose, findable by what it is ABOUT
+    #   process — a stored procedure, synced to a client as a skill
+    #   snippet — a reusable shape, with its structured fields mirrored in
+    #             `data` and its trigger composed into the title (0070)
+    #   lesson  — a transferable insight, findable by WHEN IT APPLIES rather
+    #             than by topic (milestone 385). Same mirror discipline as a
+    #             snippet: the trigger lives in `data.when_to_apply` and is
+    #             composed into the title and the head of the body, which is
+    #             what puts it in the embedded document. A lesson is never a
+    #             task — see `status` — because a lesson that acquired one
+    #             would start appearing in open-work listings.
+    #
+    # DELIBERATELY UNGATED. Unlike `task_kind` there is no CHECK on this column:
+    # migration 0036 added it as plain Text with a server default and nothing
+    # has constrained it since, so rule 36 has no whitelist to expand when a
+    # kind is added. The vocabulary that actually decides what a reader can
+    # reach is `services.knowledge._FACETS`; an unrecognised value there
+    # resolves to a filter matching nothing, which is the intended answer to a
+    # typo.
     note_type: Mapped[str] = mapped_column(Text, default="note", server_default="note")
     # Task sub-kind — what KIND of work this is, not how it is going:
     #   work   (default) — ships a change