docs: fix drift — sync compose/volumes, studio coverage, mempalace link

Audit found README/AGENTS carried a stale compose/volume set that
diverged from the shipped docker-compose.yml (DOCKER_HUB + compose +
.env.example were already consistent — README was the outlier):

- README compose block + 'Volumes and persistence' table: correct volume
  names (devbox-shell-history not -bash-history; devbox-uv at
  ~/.local/share/uv not devbox-uv-tools at /opt/uv-tools — the latter
  would SHADOW the baked mempalace install at UV_TOOL_DIR); add
  devbox-ssh-local + devbox-zoxide; mark devbox-palace/-chroma-cache
  optional; WORKSPACE_PATH/SSH_KEY_PATH (not HOST_WORKSPACE).
- README quickstart: 'compose exec -u developer' (no USER in image; bare
  exec lands a root shell).
- README: pi-studio now 'shipped' not 'planned'; build-pipeline + tag
  table cover -studio + smoke-studio/build-variant-studio.
- AGENTS: backward-compat volume names corrected; repo-layout bullets
  cover pi-studio install + studio-expose + STUDIO_EXPOSE bridge.
- DOCKER_HUB: MemPalace source link -> upstream MemPalace/mempalace
  (matches Dockerfile.base + CHANGELOG refs).

Note: the shipped v1.0.0 CHANGELOG migration note still lists the old
(incorrect) volume names; left as immutable released history.

DOCKER_HUB.md

@@ -151,7 +151,7 @@ Optional volumes for MemPalace (commented out by default — uncomment in `docke
 - **pi**: https://github.com/earendil-works/pi
 - **pi-toolkit**: https://gitea.jordbo.se/joakimp/pi-toolkit
 - **pi-extensions**: https://gitea.jordbo.se/joakimp/pi-extensions
-- **MemPalace**: https://github.com/joakimp/mempalace
+- **MemPalace**: https://github.com/MemPalace/mempalace
 
 ## License