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.testSupport.ts

@@ -208,7 +208,7 @@ export function fakeRuntime(sessionId = "session-1", patch: Partial<TestSession>
     setSessionName: (name: string) => { session.sessionName = name; },
     compact: () => Promise.resolve({ summary: "", tokensBefore: 0 }),
     getUserMessagesForForking: () => [],
-    agent: { streamFn: () => { throw new Error("streamFn should not be called in this test"); } },
+    agent: { streamFunction: () => { throw new Error("streamFunction should not be called in this test"); } },
     ...patch,
   };
   const runtime: PiSessionRuntime = {