feat: implement model persistence with per-session overrides

- Add session_config SQLite table for per-session settings
- Update routing to support session override → agent config → global default resolution chain
- Upgrade WebChat SessionBridge from NativeAgent to AgentOrchestrator
- Add /model, /local, /cloud commands to Telegram adapter
- Add /model command to WebChat gateway handlers
- Clear session overrides on /reset command
- Pass memoryStore and config through to SessionBridge
- Add comprehensive tests for all new functionality

Fixes model persistence bug where TUI model changes didn't affect WebChat/Telegram sessions. Now:
- TUI /model sets global default (persists across restarts, affects all new sessions)
- WebChat/Telegram /model sets session override (only that conversation, cleared on /reset)
- WebChat sessions gain AgentOrchestrator features (delegation, compaction, memory)

src/daemon/index.ts

@@ -120,7 +120,7 @@ export async function startDaemon(config: Config): Promise<DaemonContext> {
 
   const gateway = createGateway({
     config, sessionManager, modelRouter, systemPrompt, toolRegistry, toolExecutor,
-    channelRegistry, pairingManager, lifecycle,
+    channelRegistry, pairingManager, lifecycle, memoryStore,
     getChannelAgents: () => channelAgents,
   });