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.

AGENTS.md

@@ -47,8 +47,8 @@ When bumping the opencode version, also bump `OPENCODE_VERSION` in `Dockerfile`
 - **MemPalace install path** — installed via `uv tool install` into `/opt/uv-tools/mempalace/`. Both the `mempalace` CLI and the `mempalace-mcp` MCP server binary are shipped as entry points by the mempalace package itself and placed on PATH by uv as shims whose shebangs point at the venv's Python. No hand-rolled wrapper is needed. Do not use `pip install --break-system-packages` — that was the previous approach and has been removed. Do not use `["python3", "-m", "mempalace.mcp_server"]` in `opencode.jsonc` — system Python can't import from the uv venv.
 - **generate-config.py idempotency** — the script MUST never overwrite an existing `opencode.jsonc` or legacy `opencode.json`. Config persists in the `devbox-opencode-config` named volume; accidentally clobbering that file would destroy hand-edits. The smoke test asserts this.
 - **Skillset auto-deploy** — on every container start, `entrypoint-user.sh` looks for a skillset repo (detection order: `$SKILLSET_CONTAINER_PATH` → `$HOME/skillset` → `/workspace/skillset`) and runs `deploy-skills.sh --bootstrap --prune-stale`. This creates relative symlinks in `~/.agents/skills/` and `~/.config/opencode/instructions/`. Do NOT bind-mount `~/.agents/skills/` from the host — the container manages its own skills with relative symlinks that differ from the host's. The named volume `devbox-opencode-config` persists the deployed config across restarts.
-- **Config persistence via named volume** — `devbox-opencode-config` is a Docker named volume mounted at `~/.config/opencode/`. It is NOT a host bind mount by default. This separation allows both native and containerized opencode to coexist on the same machine without symlink conflicts. Users who need to override can replace the named volume with a host bind mount in their compose file. **Same pattern for pi:** `devbox-pi-config` is mounted at `~/.pi/` and persists user toggles (`/ext`-disabled extensions) and `~/.pi/agent/settings.json` edits across container recreate.
-- **pi install contract** — `INSTALL_PI=true` (default false) opt-in build arg. `pi` is npm-installed globally at build time; the npm prefix is NOT on a named volume, so `pi update` inside the container does not persist across `--rm` containers. Image rebuild is the upgrade path — same contract as `OPENCODE_VERSION`. The pi-toolkit and pi-extensions repos are git-cloned into `/opt/` at build time, then their `install.sh` runs from `entrypoint-user.sh` on each container start to symlink into `~/.pi/agent/` (which lives on the named volume). The mempalace pi-bridge is symlinked manually from `/opt/mempalace-toolkit/extensions/pi/mempalace.ts` — we do NOT call mempalace-toolkit's full `install.sh` because its `install_skill` step would race with skillset auto-deploy `--prune-stale`.
+- **Config persistence via named volume** — `devbox-opencode-config` is a Docker named volume mounted at `~/.config/opencode/`. It is NOT a host bind mount by default. This separation allows both native and containerized opencode to coexist on the same machine without symlink conflicts. Users who need to override can replace the named volume with a host bind mount in their compose file. **Same pattern for pi:** `devbox-pi-config` is mounted at `~/.pi/` and persists user toggles (`/ext`-disabled extensions), `~/.pi/agent/settings.json` edits, and — because `NPM_CONFIG_PREFIX` is set to `~/.pi/npm-global` — anything installed via `pi install npm:...` or `npm install -g` as the developer user, across container recreate AND image rebuild.
+- **pi install contract** — `INSTALL_PI=true` (default false) opt-in build arg. The baked `pi` binary is npm-installed globally to `/usr` at build time (system prefix). At runtime, `NPM_CONFIG_PREFIX=/home/developer/.pi/npm-global` is set in the image ENV with that prefix's `bin/` prepended to `PATH` — so any `pi install npm:...` or `npm install -g` invoked by the developer user lands on the named volume and survives everything except `docker compose down -v`. The new ENVs are declared *after* all build-time `npm install -g` calls in the Dockerfile so they don't redirect the baked installs into a path that the volume mount would later shadow. If the user runs `npm install -g @mariozechner/pi-coding-agent` themselves, the user-installed copy on the volume wins via `PATH` order; otherwise image rebuild is the upgrade path for the baked pi (same contract as `OPENCODE_VERSION`). The pi-toolkit and pi-extensions repos are git-cloned into `/opt/` at build time, then their `install.sh` runs from `entrypoint-user.sh` on each container start to symlink into `~/.pi/agent/` (which lives on the named volume). The mempalace pi-bridge is symlinked manually from `/opt/mempalace-toolkit/extensions/pi/mempalace.ts` — we do NOT call mempalace-toolkit's full `install.sh` because its `install_skill` step would race with skillset auto-deploy `--prune-stale`.
 - **Pi deploy ordering matters in entrypoint-user.sh** — `pi-toolkit` runs first (creates `keybindings.json` symlink and writes pi-env.zsh), then `pi-extensions`, then `settings.json` template bootstrap, then mempalace bridge symlink. mempalace-toolkit's `check_pi_toolkit` probe (when called from the host install path) expects keybindings to already be present — not currently called from container, but ordering matches host convention.
 - **Default CMD is `bash -l`** — not a harness. `docker compose run --rm devbox` drops the user into a login shell to choose: `aws sso login`, then `opencode` or `pi` (or any tool). Pass the harness explicitly to launch directly: `docker compose run --rm devbox opencode` / `docker compose run --rm devbox pi`. `docker compose exec` bypasses entrypoint+CMD entirely (existing user workflow unchanged).
 - **Docker Hub description update** — uses `/v2/auth/token` endpoint (not the deprecated `/v2/users/login`). Auth uses `identifier`/`secret` fields, returns `access_token`, sent as `Bearer`. Short description must be ≤100 bytes.