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.

scripts/smoke-test.sh

@@ -86,6 +86,8 @@ run "global-AGENTS append snippet present" \
     "test -f /usr/local/share/pi-devbox/pi-global-AGENTS.append.md"
 run "pi-devbox block merged into pi-global-AGENTS.md" \
     "grep -q 'pi-devbox:managed-block' /opt/pi-toolkit/pi-global-AGENTS.md"
+run "mempalace session-start pointer merged into global AGENTS.md" \
+    "grep -q 'load the mempalace skill' /opt/pi-toolkit/pi-global-AGENTS.md"
 # Vendored fallback skills (so a no-skillset container still resolves the
 # AGENTS.md 'read the pi-extensions skill' pointer).
 run "image-baked pi-extensions fallback skill" \