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.

CHANGELOG.md

@@ -15,6 +15,10 @@ Docs-only updates that don't trigger a new image build (Docker Hub description w
 - **Docs:** README pi section gains a `### Setup` paragraph mentioning the prebuilt `latest-with-pi` and `latest-omos-with-pi` Docker Hub tags, mirroring the OMOS section's `latest-omos` mention.
 - **Docs:** AGENTS.md tag-scheme paragraph corrected from "four Docker Hub tags per release" to eight (the v1.14.41b CI matrix expansion). Reclaim-disk job list updated from the four pre-pi jobs to all eight current `load: true` jobs.
 
+Image changes (will ship in the next tagged release):
+
+- **Fix:** `pi install npm:<pkg>` (and any `npm install -g`) by the `developer` user no longer EACCES against the system npm prefix. `NPM_CONFIG_PREFIX` is now `/home/developer/.pi/npm-global` and the prefix's `bin/` is prepended to `PATH`. The directory lives on the `devbox-pi-config` named volume, so user-installed pi packages (themes, skills, extensions) survive container recreation and image rebuilds. Build-time `npm install -g` calls (opencode, pi, oh-my-opencode-slim) are unaffected because the new ENVs are declared after those steps in the Dockerfile, so the baked binaries still install to `/usr` and are not shadowed by the volume mount.
+
 ## v1.14.41b — 2026-05-08
 
 **Optional pi as second harness.**