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).

package.json

@@ -83,9 +83,9 @@
   },
   "devDependencies": {
     "@changesets/cli": "^2.31.0",
-    "@earendil-works/pi-agent-core": "^0.80.8",
-    "@earendil-works/pi-ai": "^0.80.8",
-    "@earendil-works/pi-coding-agent": "^0.80.8",
+    "@earendil-works/pi-agent-core": "^0.81.1",
+    "@earendil-works/pi-ai": "^0.81.1",
+    "@earendil-works/pi-coding-agent": "^0.81.1",
     "@eslint/js": "^10.0.1",
     "@types/node": "^24.13.3",
     "@types/ws": "^8.18.1",
@@ -114,9 +114,9 @@
   "homepage": "https://pi-web.dev/",
   "packageManager": "[email protected]",
   "peerDependencies": {
-    "@earendil-works/pi-agent-core": ">=0.80.8 <0.81",
-    "@earendil-works/pi-ai": ">=0.80.8 <0.81",
-    "@earendil-works/pi-coding-agent": ">=0.80.8 <0.81"
+    "@earendil-works/pi-agent-core": ">=0.81.1 <0.82",
+    "@earendil-works/pi-ai": ">=0.81.1 <0.82",
+    "@earendil-works/pi-coding-agent": ">=0.81.1 <0.82"
   },
   "keywords": [
     "pi-package",