feat(telemetry): a usage event records which project the reader was in (#4196, #3735)

`RetrievalLog` has carried `project_id` since it existed, so "this record
was SURFACED on project B" was always answerable. `note_usage_events`
had none, so "this record was OPENED on project B" was not — and the two
cannot be joined to recover it, because there is deliberately no session
identity server-side. NoteUsageEvent's own docstring rules that out.

That gap sat exactly on the question milestone 385 exists to answer. A
lesson's whole claim is that it reaches a session on a project it was not
written on, and step 8's acceptance is "retrieved on a different project
AND opened". Each half was answerable; the conjunction was not.

WHICH project, because the name is ambiguous and the wrong reading makes
the column useless: it is the project the READER was in, never the one
the record belongs to. The record's own project is already on the note;
copying it here would answer a question nobody asked while looking like
it answered this one.

The surfacing half is free — every arm already holds the scope it just
searched, so auto_inject, lesson_slot, the write-path arms and
enter_project now record it. process_skill_sync does not and should not:
it installs every Process the operator can reach, which is not a
project-scoped question, so a project there would be a fiction.

The pull half needs the caller, since a getter knows only what it was
handed. The five single-record getters take `project_id: int = 0` and
pass it through, following the convention `search` and `create_*`
already set. Null stays an ordinary answer meaning "not reported" — a
pull with no project is still a pull and still counts toward dead
weight; it simply cannot speak to transfer. The four REST detail views
report none for now: a human opening a record in a browser is a
different event from an agent recalling one, and #2245 left that
asymmetry deliberately undecided.

Guarded the way #2245 and #2476 taught: by source inspection, because a
parameter that was never threaded through changes no return value and
shows up only as a column that is mysteriously always null. Three
guards — the signature, the pass-through, and the arms — plus the
can-fail test rule 167 asks for.

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

src/scribe/models/note_usage.py

@@ -66,12 +66,29 @@ class NoteUsageEvent(Base, CreatedAtMixin):
     # so never aggregate across the prefix without saying why (#1038, #2085).
     source: Mapped[str] = mapped_column(Text, nullable=False)
 
+    # The project the READER was in when this happened — not the project the
+    # record belongs to, which is already on `notes.project_id`. The whole
+    # point is the comparison between the two: a record opened somewhere other
+    # than where it was written is the evidence that it TRANSFERRED, which is
+    # the claim the lesson kind rests on (milestone 385) and the one thing
+    # RetrievalLog could half-answer and this table could not answer at all.
+    #
+    # Nullable, and null is ordinary rather than historical. A surfacing arm
+    # always knows the project it searched; a getter knows only what its caller
+    # passed, and a caller that passed nothing is reporting "unknown", not
+    # "none". Treat a null as unreported: it still counts toward "is this dead
+    # weight", and it cannot speak to "was it opened away from home".
+    project_id: Mapped[int | None] = mapped_column(Integer, nullable=True)
+
     __table_args__ = (
         # The readout is always "these note ids, split by event" — a covering
         # composite beats separate single-column indexes for it.
         Index("ix_note_usage_note_event", "note_id", "event"),
         Index("ix_note_usage_created_at", "created_at"),
         Index("ix_note_usage_user_id", "user_id"),
+        # note_id first, like ix_note_usage_note_event: every readout starts
+        # from a set of note ids and narrows, never from a project.
+        Index("ix_note_usage_note_project", "note_id", "project_id"),
     )
 
     def to_dict(self) -> dict: