Close inbox projection under one transaction

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.

src/main/database/migrations/v46-v46.ts

@@ -0,0 +1,44 @@
+import log from "electron-log/main.js";
+import type { Migration, MigrationDeps } from "./types";
+
+export function createMigrationsV46V46(deps: MigrationDeps): Migration[] {
+  const { db } = deps;
+  return [
+    {
+      // Ledger for `prepareRun` inbox → session-shadow projection.
+      //
+      // Before v46 the projection sequence — append to the session
+      // shadow, write the thread event, mark the inbox row delivered —
+      // ran as three independent SQL writes. A crash or worker restart
+      // between the append and the delivered mark caused the next run
+      // to replay the same inbox row, duplicating transcript turns and
+      // thread events.
+      //
+      // The ledger is keyed by the natural identity of a projection
+      // attempt: the combination of the inbox message and the session
+      // shadow it lands on. `INSERT OR IGNORE` against that key is the
+      // idempotent gate — the first insert wins, a replay silently
+      // skips. Callers wrap the ledger insert, the shadow append, the
+      // thread-event write, and the delivered-mark in a single
+      // `BEGIN IMMEDIATE` transaction so the four writes commit
+      // together or roll back together.
+      version: 46,
+      name: "agent_message_projections_ledger",
+      fn: () => {
+        db.exec(`
+          CREATE TABLE IF NOT EXISTS agent_message_projections (
+            message_id TEXT NOT NULL,
+            session_id TEXT NOT NULL,
+            projected_at TEXT NOT NULL,
+            PRIMARY KEY (message_id, session_id)
+          )
+        `);
+        db.exec(`
+          CREATE INDEX IF NOT EXISTS idx_agent_message_projections_session
+          ON agent_message_projections(session_id, projected_at)
+        `);
+        log.info("[migration v46] agent_message_projections ledger installed");
+      },
+    },
+  ];
+}