feat(lessons): a lesson reaches the moment it applies, and says it binds nothing (#3732)

WHICH ARM — the two note arms, and no new one.

`write_path` filters kinds, so a lesson was not outranked there but
unreachable, which is #3702's shape: an arm that never had the candidate
reports a healthy bar. It now asks for lessons alongside snippets and
issues. The founding example of the kind is a lesson about a code shape,
and this is the arm that fires when code is written.

`auto_inject` does not filter kinds, so lessons were already candidates —
but scoped to the bound project, which for a kind whose whole claim is
that it transfers is the same silence. Both arms now pass
`include_global_kinds` (#3730).

WHOSE BUDGET — a reserved slot in the prompt menu, none on the write path.

The step's premise needs a correction: the notes menu and the rule hints
are separate functions with separate budgets, so a line reserved here
displaces a note, never a rule. (`RULEHINT_LIMIT` is also 5, not 1, since
#4102 made it a default rather than a cap.)

The trade taken: a note crowded out is a lost convenience and a rule
crowded out still fires at an act arm, but a lesson crowded out is the
feature failing — a lesson exists only to be met at the moment it
applies, so the arm IS its delivery and the loss is total and silent.
That is `preference_slot`'s argument, and the rarity is `reuse_slot`'s.
It buys position, never a lower bar, and it EXTENDS rather than evicting:
a displaced hit sits in the general search's own log row, and evicting it
would make two tables disagree about one call (#3668, #379).

No slot on the write path: that arm fires before every Write and Edit,
where a guaranteed extra line is a guaranteed extra interruption, and its
field is already just snippets, issues and lessons rather than the whole
corpus.

`lesson_slot` logs its own retrieval and its own surfacing from the first
deploy, and the general contest stays open to the kind — otherwise "the
slot earns its line" would be true by construction.

THE VOICE — "they don't always have to be followed".

The menu's register is already the non-binding one. What it lacked is
that a lesson reads as one more title in a list of material when it is
advice someone paid for. One clause, in the header, only when a lesson is
on the menu: weigh it, use your judgement, it is not a rule and binds
nothing. It deliberately does not borrow the rule arms' "before deciding
it does not apply", and a guard asserts that phrase never appears.

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

tests/test_write_path_trigger.py

@@ -253,9 +253,11 @@ async def test_semantic_arm_is_snippet_only_and_browse_scoped():
         await pc.build_write_path_hint(1, "src/x.py", code=REAL_CODE)
     kwargs = search.await_args.kwargs
     # Prior art is snippets AND recorded experience (#2246) — an issue saying
-    # "we tried this and it broke" belongs here. What stays out is the open
-    # to-do list, which resembles the code and answers nothing.
-    assert kwargs["note_type"] == ("snippet", "note")
+    # "we tried this and it broke" belongs here — AND lessons (milestone 385
+    # step 5), whose founding example is a lesson about a code shape. What
+    # stays out is the open to-do list, which resembles the code and answers
+    # nothing.
+    assert kwargs["note_type"] == ("snippet", "note", "lesson")
     assert kwargs["task_kind"] == "issue"
     # Nobody asked for this, so it must not reach a one-to-one direct share.
     assert kwargs["scope"] == "browse"