feat(session): persist model tier overrides per session

Store per-session config in SQLite and route /model and /reset through command fast-paths so channel sessions keep independent model selection across reconnects and restarts.

src/session/index.ts

@@ -1,2 +1,6 @@
 export { SessionStore, parseDuration } from './store.js';
 export { SessionManager, ManagedSession, type Session } from './manager.js';
+export { SessionIndexer, tokenize } from './indexer.js';
+export type { HistoryMetadata, HistoryIndexerConfig } from './indexer.js';
+export { SessionSearch } from './search.js';
+export type { HistorySearchResult, HistorySearchConfig } from './search.js';