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.

.env.example

@@ -75,11 +75,15 @@ SSH_KEY_PATH=~/.ssh
 # palace path — wing data is mutually visible to either harness.
 #
 # Pi version is baked at build time via PI_VERSION (default: latest at
-# build). `pi update` inside the container would write to the npm global
-# prefix, which is not on a named volume — updates do not persist across
-# `--rm` containers. Rebuild the image to upgrade pi.
+# build). The baked `pi` binary is at /usr/bin/pi (system npm prefix);
+# rebuild the image to upgrade it. NPM_CONFIG_PREFIX is set to
+# /home/developer/.pi/npm-global, so anything installed via
+# `pi install npm:...` or `npm install -g` as the developer user
+# (themes, skills, extensions, including a user-installed pi itself)
+# lands on the named volume and survives container recreate AND image
+# rebuilds. A user-installed pi wins via PATH order over the baked one.
 #
-# Pi config (settings.json, extensions toggle state) persists in the
+# Pi config (settings.json, extensions toggle state, sessions, auth) persists in the
 # devbox-pi-config named volume mounted at ~/.pi/.
 #
 # To launch pi from a `compose run` invocation: