feat(sessions): reject extension-scoped provider registrations

PI WEB only supports globally configured providers (Pi built-ins,
agent-dir models.json, environment credentials). A daemon-wide shim on
the shared ModelRuntime swallows extension registerProvider calls and
makes unregisterProvider a no-op, so one workspace's extensions can no
longer corrupt the provider set of concurrent sessions (issue #76).

Rejections during a services load surface as session warnings through
the existing diagnostics pipeline; late registrations from session
event handlers broadcast a notification to active sessions. Everything
else extensions register keeps working.

Requires manual restart of pi-web-sessiond.service (daemon wiring changed).

src/server/sessiond.ts

@@ -7,6 +7,7 @@ import { WorkspaceActivityService } from "./activity/workspaceActivityService.js
 import { registerWorkspaceActivityRoutes } from "./activity/workspaceActivityRoutes.js";
 import { SessionEventHub } from "./realtime/sessionEventHub.js";
 import { AuthService } from "./sessions/authService.js";
+import { installGlobalProviderPolicy } from "./sessions/globalProviderPolicy.js";
 import { registerAuthRoutes } from "./sessions/authRoutes.js";
 import { PiSessionService } from "./sessions/piSessionService.js";
 import { createPiSessionManagerGateway } from "./sessions/piSessionManagerGateway.js";
@@ -69,6 +70,9 @@ await runSessionDaemonStartup({
       }),
     });
     auth.subscribe((change) => { sessions.applyAuthChange(change); });
+    // PI WEB only supports globally configured providers: reject every
+    // extension provider registration against the shared daemon-wide runtime.
+    installGlobalProviderPolicy(auth.runtime, (providerId) => { sessions.noteRejectedProviderRegistration(providerId); });
     const terminals = new TerminalService(eventHub, workspaceActivity);
     const runtimeComponent = Object.freeze({
       ...getPiWebRuntimeComponent("sessiond", SESSIOND_RUNTIME_CAPABILITIES),