feat(uninstall): one-command full uninstall with confirmation

- uninstall.sh: removes pi-kit from settings.json (string + gated object
  entries), the package clone, kit-installed public skills, ~/.config/pi-kit
  (env.sh with tokens, audit.jsonl) and the shell-rc sourcing block
- confirmation prompt by default; PI_KIT_UNINSTALL=1 for non-interactive,
  PI_KIT_KEEP_CONFIG=1 / PI_KIT_KEEP_SKILLS=1 to retain parts
- works without the pi binary (settings edited directly); notes how to
  remove pi itself
- tests: test/uninstall.test.sh against a throwaway $HOME (14 checks:
  full/partial removal, refusal without flag, idempotent re-run), wired
  into npm test

package.json

@@ -28,7 +28,7 @@
   },
   "scripts": {
     "typecheck": "tsc --noEmit",
-    "test": "node --experimental-strip-types test/guardrails.test.ts && bash test/shell.test.sh"
+    "test": "node --experimental-strip-types test/guardrails.test.ts && bash test/shell.test.sh && bash test/uninstall.test.sh"
   },
   "peerDependencies": {
     "@earendil-works/pi-ai": "*",