When every row create of a turn loses to the SQLite lock, the queued token delta's
"ensure the row exists" guard becomes the session's first writer and minted the row as
source='unknown'. That placeholder was permanent on the real path even with the upsert
repair from #112045: the turn lease (turn_facade_lease.admit_durable_turn) treats an existing
row as proof the create already happened and sets _session_db_created, so the creator never
returns to repair it. Live probe: a platform="desktop" AIAgent whose create_session raised
"database is locked" for the whole first turn ended with a source='unknown' row on base AND
on the contributor head; with this change the row is minted 'desktop' by the guard itself.
Producer fix: update_token_counts gains an optional source= that the two agent call sites
(agent/turn_usage.py, agent/codex_runtime.py) fill from _session_source_for_agent(platform),
the same value _ensure_db_session would stamp. record_auxiliary_usage has no surface and
keeps the placeholder, which the creator's upsert now repairs.
Salvage trims: the contributor's SimpleNamespace dispatch test is replaced by a real-AIAgent
invariant test under tests/agent/ (the dispatch hunk in _run_prompt_submit is kept; the
INSERT-OR-IGNORE is idempotent under prompt.submit's own persist); narration comments cut
to the WHY; docs list 'unknown' among the startup-sweep sources.
Refs #111999
cfd752e6f7