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/channels/telegram/adapter.test.ts

@@ -62,10 +62,13 @@ describe('TelegramAdapter', () => {
 
     // .use() for auth middleware
     expect(mockUse).toHaveBeenCalledTimes(1);
-    // .command() for /start and /reset
-    expect(mockCommand).toHaveBeenCalledTimes(2);
+    // .command() for /start, /reset, /model, /local, /cloud
+    expect(mockCommand).toHaveBeenCalledTimes(5);
     expect(mockCommand.mock.calls[0][0]).toBe('start');
     expect(mockCommand.mock.calls[1][0]).toBe('reset');
+    expect(mockCommand.mock.calls[2][0]).toBe('model');
+    expect(mockCommand.mock.calls[3][0]).toBe('local');
+    expect(mockCommand.mock.calls[4][0]).toBe('cloud');
     // .on('message:text', ...) for text handler
     expect(mockOn).toHaveBeenCalledWith('message:text', expect.any(Function));
     // .start() to begin long polling