feat(sessions): allow global-extension providers, require Pi 0.81

Relaxes the provider policy from 'global config only' to 'global sources':
providers registered by agent-dir (global) extensions are learned once at
daemon startup and allowed on the shared runtime; project-extension
registrations are still rejected with a session warning. Global extensions
load identically for every session, so their providers are daemon-consistent
and cannot leak project state (#76).

- Shim now allows allowlisted ids through and also covers Pi 0.81's native
  provider path (registerNativeProvider), closing a bypass.
- Startup learning step loads only global extensions against a scratch cwd
  and diffs the runtime's registered provider ids.
- Bumps @earendil-works/* dev/peer ranges to >=0.81.1 <0.82; adapts to the
  Agent.streamFn -> streamFunction rename.
- Docs, changeset, unit and acceptance tests updated (global-extension allow
  path, late re-registration a la pi-tensorx, native provider rule).

src/server/sessions/piSessionService.promptQueue.test.ts

@@ -99,7 +99,7 @@ describe("PiSessionService prompt, queue, and auth warnings", () => {
       return stream;
     };
     const hub = new CapturingSessionEventHub();
-    const fake = fakeRuntime("name-session", { model, agent: { streamFn } });
+    const fake = fakeRuntime("name-session", { model, agent: { streamFunction: streamFn } });
     const service = new PiSessionService(hub, {
       agentDir: TEST_AGENT_DIR,
       modelRuntime: testModelRuntime,