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/migrations/v49-v49.ts

@@ -0,0 +1,40 @@
+import log from "electron-log/main.js";
+import type { Migration, MigrationDeps } from "./types";
+import { columnExists, tableExists } from "./types";
+
+/**
+ * Drop the unused `agent_messages.read_at` column.
+ *
+ * The column was added in v11 when reads were tracked per-message on a
+ * single shared receipt. v12 moved read tracking to the
+ * `agent_message_reads` join table, which is now the sole source of
+ * truth: every consumer that surfaces `readAt` (renderer snapshot,
+ * per-agent inbox, board receipts) resolves it from that table.
+ * `agent_messages.read_at` has not been written by any production path
+ * for multiple releases; leaving it around just forced every SELECT to
+ * carry a NULL column through wire-shape layers and invited the next
+ * maintainer to accidentally read stale data from it.
+ */
+export function createMigrationsV49V49(deps: MigrationDeps): Migration[] {
+  const { db } = deps;
+  return [
+    {
+      version: 49,
+      name: "drop_agent_messages_read_at",
+      fn: () => {
+        if (!tableExists(db, "agent_messages")) {
+          log.info("[migration v49] agent_messages table missing; skipping");
+          return;
+        }
+        if (!columnExists(db, "agent_messages", "read_at")) {
+          log.info("[migration v49] agent_messages.read_at already absent; skipping");
+          return;
+        }
+        db.exec("ALTER TABLE agent_messages DROP COLUMN read_at");
+        log.info(
+          "[migration v49] dropped unused agent_messages.read_at column — reads are tracked per-reader in agent_message_reads",
+        );
+      },
+    },
+  ];
+}