feat(skills): add mempalace proactive-load directive for containers

Baking the mempalace fallback skill fixed *availability*, but mempalace had
no proactive-load directive anywhere (pi-toolkit's global AGENTS.md only
points to pi-extensions), so a new container would still surface it only via
description-matching — the same under-utilisation the pi-extensions directive
was created to fix.

Add a session-start pointer to the pi-devbox managed AGENTS.md block
(pi-global-AGENTS.append.md): gated to pi-devbox containers and conditional on
the MemPalace MCP tools being present. Memory continuity matters most in a
frequently-recreated container — the palace is its only cross-recreate memory.

- pi-global-AGENTS.append.md: '## Session start: load the mempalace skill'.
- smoke-test: assert the pointer merges into the global AGENTS.md at build.
- docs: VENDORED.md, README, CHANGELOG [Unreleased].

Now both skills are complete in pi-devbox: directive + skill file.
pi-extensions = directive (pi-toolkit) + baked skill; mempalace = directive
(this block) + baked skill.

README.md

@@ -470,7 +470,10 @@ directory, and they compose:
   the pinned `pi-extensions` clone (`/opt/pi-extensions/skill/`) over it at
   build, so a normal build ships the fresh copy and an old-ref/mirror build
   still ships the snapshot. `mempalace` is snapshot-only (its consumer skill
-  has no public package home). See
+  has no public package home), and because pi-toolkit's `AGENTS.md` has no
+  directive for it, the pi-devbox managed block adds a session-start
+  *proactive-load* pointer for it (gated to pi-devbox containers, conditional
+  on the MemPalace MCP tools) so a new container actually loads it. See
   `rootfs/usr/local/share/pi-devbox/skills/VENDORED.md`.
 - **Skillset repo (optional).** If a `skillset` repo is mounted (at
   `$HOME/skillset` or `/workspace/skillset`, or via `SKILLSET_CONTAINER_PATH`),