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/services/snippets.py

@@ -55,10 +55,15 @@ UNSET: object = object()
 # --- serialize: structured fields -> note (title/body/tags) ------------------
 
 def compose_title(name: str, when_to_use: str = "") -> str:
-    """`name — when to use` (or just `name` when no usage note is given)."""
-    name = (name or "").strip()
-    when = (when_to_use or "").strip()
-    return f"{name} — {when}" if when else name
+    """`name — when to use` (or just `name` when no usage note is given).
+
+    The join itself lives in `embeddings.trigger_title`, which rules and
+    lessons build their titles from too. Kept as a named function here because
+    it is this module's public vocabulary and callers say `compose_title`.
+    """
+    from scribe.services.embeddings import trigger_title
+
+    return trigger_title(name, when_to_use)
 
 
 def compose_tags(language: str = "", tags: list[str] | None = None) -> list[str]: