The `agent_messages.read_at` column has had no production writer since
v12 moved read tracking into the per-viewer `agent_message_reads`
receipts table. It survived anyway, carrying a NULL through every SELECT
list and begging the next maintainer to accidentally read stale data
from it.
- v49 migration: `ALTER TABLE agent_messages DROP COLUMN read_at`
(idempotent: skips if absent).
- messages.ts: SELECTs no longer fetch the column; per-viewer `readAt`
is computed from `agent_message_reads` as before, with `null` for
call-sites that have no viewer context (getAgentMessage /
searchMessages / listAgentInboxUnread).
- runtime-bridge.ts / runtime-message-injector.ts: comments referencing
the dropped column updated to name the read-receipts table instead.
ZacharyZhang-NY
d7e267031f