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/handlers/agent.ts

@@ -5,17 +5,20 @@ import type { SessionBridge } from '../session-bridge.js';
 import type { LaneQueue } from '../lane-queue.js';
 import type { MetricsCollector } from '../metrics.js';
 import type { Attachment } from '../../channels/types.js';
+import type { SessionManager } from '../../session/manager.js';
+import type { ModelTier } from '../../models/router.js';
 
 export interface AgentHandlerDeps {
   sessionBridge: SessionBridge;
   laneQueue: LaneQueue;
   metrics?: MetricsCollector;
+  sessionManager?: SessionManager;
 }
 
 export function createAgentHandlers(deps: AgentHandlerDeps) {
   return {
     'agent.send': async (request: GatewayRequest, send: SendFn): Promise<OutboundMessage | void> => {
-      const params = request.params as { message?: string; connectionId?: string; attachments?: GatewayAttachment[]; metadata?: { isCommand?: boolean; command?: string } } | undefined;
+      const params = request.params as { message?: string; connectionId?: string; attachments?: GatewayAttachment[]; metadata?: { isCommand?: boolean; command?: string; commandArgs?: string } } | undefined;
       if (!params?.message && !params?.metadata?.isCommand) {
         return makeError(request.id, ErrorCode.InvalidRequest, 'message is required');
       }
@@ -48,9 +51,51 @@ export function createAgentHandlers(deps: AgentHandlerDeps) {
           try {
             if (params.metadata.command === 'reset') {
               agent.reset();
+              // Clear session config
+              const sessionId = deps.sessionBridge.getSessionId(connectionId);
+              if (sessionId && deps.sessionManager) {
+                deps.sessionManager.deleteSessionConfig('ws', sessionId, 'modelTier');
+              }
               send(makeEvent(request.id, 'done', { content: 'Session reset.' }));
               return;
             }
+
+            if (params.metadata.command === 'model') {
+              const modelArg = params.metadata.commandArgs as string | undefined;
+              const sessionId = deps.sessionBridge.getSessionId(connectionId);
+
+              if (!modelArg) {
+                // Show current tier info
+                const currentTier = agent.getModelTier();
+                send(makeEvent(request.id, 'done', {
+                  content: `Current model tier: ${currentTier}`,
+                }));
+                return;
+              }
+
+              // Validate tier
+              const validTiers: ModelTier[] = ['fast', 'default', 'complex', 'local'];
+              const tier = modelArg as ModelTier;
+              if (!validTiers.includes(tier)) {
+                send(makeEvent(request.id, 'done', {
+                  content: `Invalid tier: ${modelArg}. Available: ${validTiers.join(', ')}`,
+                }));
+                return;
+              }
+
+              // Update agent tier
+              agent.setModelTier(tier);
+
+              // Persist to session config
+              if (sessionId && deps.sessionManager) {
+                deps.sessionManager.setSessionConfig('ws', sessionId, 'modelTier', tier);
+              }
+
+              send(makeEvent(request.id, 'done', {
+                content: `Switched to model tier: ${tier}`,
+              }));
+              return;
+            }
           } finally {
             deps.sessionBridge.setBusy(connectionId, false);
             deps.metrics?.endRequest(requestId);