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/session/manager.ts

@@ -8,6 +8,9 @@ export interface Session {
   getHistory(): Message[];
   clear(): void;
   replaceHistory(messages: Message[]): void;
+  getConfig(key: string): string | undefined;
+  setConfig(key: string, value: string): void;
+  deleteConfig(key: string): void;
 }
 
 export class ManagedSession implements Session {
@@ -64,6 +67,18 @@ export class ManagedSession implements Session {
   setHistory(messages: Message[]): void {
     this.history = [...messages];
   }
+
+  getConfig(key: string): string | undefined {
+    return this.store.getSessionConfig(this.id, key);
+  }
+
+  setConfig(key: string, value: string): void {
+    this.store.setSessionConfig(this.id, key, value);
+  }
+
+  deleteConfig(key: string): void {
+    this.store.deleteSessionConfig(this.id, key);
+  }
 }
 
 export class SessionManager {
@@ -129,4 +144,22 @@ export class SessionManager {
       this.sessions.delete(id);
     }
   }
+
+  /** Get a session config value. */
+  getSessionConfig(frontend: string, userId: string, key: string): string | undefined {
+    const session = this.getSession(frontend, userId);
+    return session.getConfig(key);
+  }
+
+  /** Set a session config value. */
+  setSessionConfig(frontend: string, userId: string, key: string, value: string): void {
+    const session = this.getSession(frontend, userId);
+    session.setConfig(key, value);
+  }
+
+  /** Delete a session config value. */
+  deleteSessionConfig(frontend: string, userId: string, key: string): void {
+    const session = this.getSession(frontend, userId);
+    session.deleteConfig(key);
+  }
 }