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>

src/db/schema.ts

@@ -140,4 +140,13 @@ CREATE TABLE processing_ack (
   status         TEXT NOT NULL,
   status_changed TEXT NOT NULL
 );
+
+-- Persistent key/value state owned by the container. Used (among other things)
+-- to store the SDK session ID so the agent's conversation resumes across
+-- container restarts. Cleared by /clear.
+CREATE TABLE session_state (
+  key        TEXT PRIMARY KEY,
+  value      TEXT NOT NULL,
+  updated_at TEXT NOT NULL
+);
 `;