feat(skills): bake pi-extensions + mempalace fallback skills

The pi-toolkit global AGENTS.md tells every pi session to read
~/.agents/skills/pi-extensions/SKILL.md at start (the fork/recall
under-utilisation fix), but that skill lived only in the private skillset
repo — so the pointer dangled in any container started without skillset
mounted. Bake fallbacks so the pointer always resolves.

- pi-extensions (Option 1 + Option 2, layered):
  * Canonical skill promoted to the public pi-extensions package repo under
    skill/ (separate commit there); co-located with the code it documents.
  * rootfs/ carries a committed snapshot (the floor).
  * Dockerfile.variant copies /opt/pi-extensions/skill/ over the snapshot
    after the pinned clone, so a normal build ships the fresh package copy
    (recorded via PI_EXTENSIONS_REF) and an old-ref/mirror build still ships
    the snapshot. Helper evaluate-extension-usage.py travels with it.
- mempalace (Option 2 only): snapshot in rootfs/. Its consumer skill has no
  public package home (mempalace-toolkit ships a different skill,
  opencode-mempalace-bridge), so no build-time refresh.
- entrypoint links both (only-when-absent; mounted skillset still wins).
- smoke-test: build-time presence + package-match check + runtime symlink
  assertions; readiness gate now waits on the last-linked skill.
- docs: skills/VENDORED.md (provenance + refresh), README, AGENTS.md,
  CHANGELOG [Unreleased].

Note: shipped in the NEXT release; v1.2.0 (run 409) predates this.

AGENTS.md

@@ -17,7 +17,10 @@ re-brand of opencode-devbox's `pi-only` variant.
   (`-studio` variant). Also appends the pi-devbox managed block from
   `pi-global-AGENTS.append.md` onto pi-toolkit's `pi-global-AGENTS.md` (the
   single global instruction slot pi loads) so containers proactively load the
-  baked `pi-devbox-environment` skill. Idempotent via a marker grep.
+  baked `pi-devbox-environment` skill. Idempotent via a marker grep. After the
+  pinned clones it also refreshes the vendored `pi-extensions` fallback skill
+  by copying `/opt/pi-extensions/skill/` over the committed `rootfs/` snapshot
+  (Option 1 over Option 2 — see `skills/VENDORED.md`).
 - `entrypoint.sh` — UID/GID alignment as root, then drops to `developer`.
 - `entrypoint-user.sh` — per-container start: SSH ControlMaster socket
   dir, LAN-access setup, MemPalace init, pi-toolkit + pi-extensions
@@ -27,8 +30,10 @@ re-brand of opencode-devbox's `pi-only` variant.
 - `rootfs/` — files baked into the image (bash aliases, inputrc,
   setup-lan-access.sh, `studio-expose` helper). Also
   `usr/local/share/pi-devbox/skills/<name>/SKILL.md` — image-baked agent
-  skills (e.g. `pi-devbox-environment`) symlinked into `~/.agents/skills/` by
-  the entrypoint, available with or without a mounted skillset — plus
+  skills (the repo-authored `pi-devbox-environment`, plus vendored fallback
+  copies of `pi-extensions` and `mempalace` — see `skills/VENDORED.md`)
+  symlinked into `~/.agents/skills/` by the entrypoint, available with or
+  without a mounted skillset — plus
   `usr/local/share/pi-devbox/pi-global-AGENTS.append.md` (the global-AGENTS
   pointer concatenated in `Dockerfile.variant`).
 - `scripts/smoke-test.sh` — sanity checks run by CI before pushing to Hub.