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/gateway/server.ts

@@ -32,6 +32,7 @@ import type { ToolExecutor } from '../tools/executor.js';
 import type { WebhookHandler } from '../automation/webhooks.js';
 import type { GmailWatcher } from '../automation/gmail.js';
 import type { PairingManager } from '../channels/pairing.js';
+import type { MemoryStore } from '../memory/store.js';
 
 export interface GatewayServerConfig {
   port: number;
@@ -60,6 +61,7 @@ export interface GatewayServerConfig {
   getTokenUsage?: () => TokenUsageEntry[];
   /** Optional pairing manager for DM pairing code management via gateway. */
   pairingManager?: PairingManager;
+  memoryStore?: MemoryStore;
 }
 
 export class GatewayServer {
@@ -82,6 +84,8 @@ export class GatewayServer {
       systemPrompt: config.systemPrompt,
       toolRegistry: config.toolRegistry,
       toolExecutor: config.toolExecutor,
+      config: config.config,
+      memoryStore: config.memoryStore,
     });
 
     this.laneQueue = new LaneQueue();
@@ -127,6 +131,7 @@ export class GatewayServer {
       sessionBridge: this.sessionBridge,
       laneQueue: this.laneQueue,
       metrics: this.metrics,
+      sessionManager: this.config.sessionManager,
     });
 
     // Config handlers (only if config object is provided)