Fix: developer-writable npm prefix for pi install

NPM_CONFIG_PREFIX is now /home/developer/.pi/npm-global, with that
prefix's bin/ prepended to PATH. Without this, 'pi install npm:<pkg>'
(and any 'npm install -g') by the developer user would EACCES against
the system prefix (/usr).

The new prefix lives on the devbox-pi-config named volume, so:
  - User-installed pi packages (themes, skills, extensions) survive
    container recreate AND image rebuild, complementing pi's auto-
    restore from settings.json with one less cold-start step.
  - A user-driven 'npm install -g @mariozechner/pi-coding-agent' lands
    on the volume and wins over the baked pi via PATH order.

Build-time 'npm install -g' calls (opencode, pi, oh-my-opencode-slim)
are unaffected: the new ENVs are declared after those steps in the
Dockerfile, so the baked binaries still install to /usr at build time
and are not shadowed by the volume mount at runtime.

Verified end-to-end with a Bun-driven smoke test: as developer,
'npm install -g cowsay' inside the container succeeds, the binary
lands on PATH, and survives a fresh container against the same volume.

DOCKER_HUB.md regenerated (24997/25000 bytes, 3-byte headroom — was
138 before; future README additions to the persistence section need
to trim something else first).

Docs updated: Dockerfile inline comments, README persistence section,
AGENTS install contract, DOCKER_HUB persistence table, .env.example
notes, CHANGELOG Unreleased entry.

DOCKER_HUB.md

@@ -375,10 +375,10 @@ pi ships pre-wired with the mempalace bridge — the `mempalace.ts` extension is
 
 | Path in container | Volume | Contains |
 |---|---|---|
-| `/home/developer/.pi` | `devbox-pi-config` (default) | `settings.json`, `/ext`-disabled extensions, pi user state |
-| `/home/developer/.mempalace` | `devbox-palace` (uncomment to enable) | Shared palace — visible to both pi and opencode |
+| `/home/developer/.pi` | `devbox-pi-config` (default) | `settings.json`, `agent/extensions/`, `auth.json`, `sessions/`, plus `git/` and `npm-global/` (`NPM_CONFIG_PREFIX` points here) so `pi install npm:` / `git:` and `npm i -g` survive container recreate and image rebuild. |
+| `/home/developer/.mempalace` | `devbox-palace` (uncomment) | Shared palace — visible to pi and opencode |
 
-The `pi` binary, pi-toolkit, and pi-extensions are baked into the image; `pi update` does **not** persist across `--rm` containers — image rebuild is the upgrade path.
+Baked pi (`/usr/bin/pi`, `/opt/pi-*`) ships in the image; rebuild to upgrade. `npm i -g @mariozechner/pi-coding-agent` lands on the volume and wins via `PATH`.
 
 Full build args, extension list, and toolkit detail: <https://gitea.jordbo.se/joakimp/opencode-devbox#pi-alternativecomplementary-harness>