fix(v2): persist SDK session ID across container restarts

The v2 poll loop held the session ID in a local variable, so every
container restart started a fresh SDK session even though the .jsonl
transcript was still sitting in the shared .claude mount. Store it in
outbound.db (container-owned, already per channel/thread), seed the
loop on startup, clear on /clear, and recover from stale-session
errors the same way v1 did.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/db/session-state.ts

@@ -0,0 +1,41 @@
+/**
+ * Persistent key/value state for the container. Lives in outbound.db
+ * (container-owned, already scoped per channel/thread).
+ *
+ * Primary use: remember the SDK session ID so the agent's conversation
+ * resumes across container restarts. Cleared by /clear.
+ */
+import { getOutboundDb } from './connection.js';
+
+const SDK_SESSION_KEY = 'sdk_session_id';
+
+function getValue(key: string): string | undefined {
+  const row = getOutboundDb()
+    .prepare('SELECT value FROM session_state WHERE key = ?')
+    .get(key) as { value: string } | undefined;
+  return row?.value;
+}
+
+function setValue(key: string, value: string): void {
+  getOutboundDb()
+    .prepare(
+      'INSERT OR REPLACE INTO session_state (key, value, updated_at) VALUES (?, ?, ?)',
+    )
+    .run(key, value, new Date().toISOString());
+}
+
+function deleteValue(key: string): void {
+  getOutboundDb().prepare('DELETE FROM session_state WHERE key = ?').run(key);
+}
+
+export function getStoredSessionId(): string | undefined {
+  return getValue(SDK_SESSION_KEY);
+}
+
+export function setStoredSessionId(sessionId: string): void {
+  setValue(SDK_SESSION_KEY, sessionId);
+}
+
+export function clearStoredSessionId(): void {
+  deleteValue(SDK_SESSION_KEY);
+}