Drop unused agent_messages.read_at column via migration v49 (M1)

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.

src/main/database/snapshot.ts

@@ -41,8 +41,9 @@ export function buildSnapshotMethods(
     // view has exactly one human viewer (the Board operator), so the
     // renderer snapshot fills it from that reader's agent_message_reads
     // rows. Agent-side queries (listAgentMessages) resolve it against
-    // the current agent's receipts. The raw agent_messages.read_at
-    // column is never read on any path and stays NULL by design.
+    // the current agent's receipts. The old `agent_messages.read_at`
+    // column was dropped in migration v49 — the join table is the
+    // sole source of truth.
     const boardReadReceiptMap = new Map(
       readAll<{ messageId: string; readAt: string }>(
         `select reads.message_id as messageId, reads.read_at as readAt
@@ -183,10 +184,9 @@ export function buildSnapshotMethods(
         createdAt: r.created_at, updatedAt: r.updated_at,
       })),
       agentMessages: readAll<SqliteRow>(
-        // The raw `read_at` column is intentionally omitted from the
-        // SELECT list. It is never written — readAt semantics live in
-        // the agent_message_reads receipts table, resolved against the
-        // current viewer above.
+        // readAt semantics live in the agent_message_reads receipts
+        // table, resolved against the current viewer above. The old
+        // `agent_messages.read_at` column was dropped in v49.
         `select id, company_id, from_agent_id, to_agent_id, sender_kind, channel, channel_target_id, subject, body, priority, parent_message_id, attachments_json, created_at from agent_messages${companyWhere} order by created_at desc limit 1000`,
         ...companyArgs,
       ).map((r) => ({