The old `prepareRun` projection drifted: append-to-shadow, thread-event,
and `markAgentInboxDelivered` ran as three independent writes, so a
crash between the append and the delivered mark replayed the same
inbox row on the next run and double-populated the transcript.
v46 installs `agent_message_projections` keyed by (message_id,
session_id) as the idempotent gate. The projection closure now wraps
the ledger claim, the shadow append, the thread-event write, and the
delivered-mark inside one `BEGIN IMMEDIATE` transaction (exposed via
`AppDatabase.withTransaction`). `claimProjection` returns false on a
replay, so the whole closure short-circuits — no double transcript,
no double thread event.
