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/backends/native/agent.test.ts

@@ -56,6 +56,9 @@ describe('NativeAgent', () => {
       addMessage: vi.fn(),
       clear: vi.fn(),
       replaceHistory: vi.fn(),
+      getConfig: vi.fn().mockReturnValue(undefined),
+      setConfig: vi.fn(),
+      deleteConfig: vi.fn(),
     };
 
     const agent = new NativeAgent({