feat(entrypoint): link ~/.pi/agent/models.json to a mounted shared file

pi reads custom providers from ~/.pi/agent/models.json, and ~/.pi is the
devbox-pi-config named volume. A file baked into the image would only seed a
fresh volume, once. A hand-made link is one more step on every new volume: on
2026-10-12 that was the same `ln -s /opt/fleet-pi/models.json …` once per
device, for one fleet-shared file.

New rootfs/usr/local/lib/pi-devbox/link-pi-models.sh, called from
entrypoint-user.sh after the pi-toolkit and pi-extensions installs. When
/opt/fleet-pi/models.json exists (PI_MODELS_JSON overrides the path, =off
disables it), it links ~/.pi/agent/models.json to it, honouring
PI_CODING_AGENT_DIR. It only ever creates a link where nothing exists: a real
file or a link of the user's own is left alone, with one NOTE line. With
nothing mounted, it does nothing and prints nothing. A link whose target later
vanishes is harmless; measured with pi 1.0.0, a dangling models.json behaves
like a missing one (rc 0, no error, no custom models).

Tests:
- scripts/test-link-pi-models.sh: 13 two-sided cases against a throwaway HOME.
  Wired into lint.yml, so it runs on every push and not only at release.
- Of 11 mutations, 10 are caught. The 11th (ln -s -> ln -sfn on the final
  link) is unreachable behind the guards.
- smoke-test.sh: with nothing mounted no link appears, and the script links a
  mounted file. Both commands were run under dash here.
- recreate-sanity-check.sh: pass, warn or fail on the link, but only when a
  shared file is mounted. Here it passes.

Docs:
- README "Custom model providers (models.json)": mount the DIRECTORY (git pull
  renames, and a single-file mount then shows "No such file" until recreate),
  keep keys in .env as "$NAME".
- .env.example: PI_MODELS_JSON.

Local gates green: lint-shell, check-workflow-shell, check-dockerfile-directives,
check-base-hash, check-doc-drift.

scripts/smoke-test.sh

@@ -672,6 +672,8 @@ exec_test "keybindings.json (pi-toolkit)"         'test -L $HOME/.pi/agent/keybi
 exec_test "extensions ≥ 4 (pi-extensions)"        'count=$(ls -1 $HOME/.pi/agent/extensions/*.ts 2>/dev/null | wc -l); [ $count -ge 4 ] && echo "$count extensions"'
 exec_test "mempalace.ts bridge"                    'test -L $HOME/.pi/agent/extensions/mempalace.ts && echo ok'
 exec_test "settings.json bootstrapped"             'test -f $HOME/.pi/agent/settings.json && echo ok'
+exec_test "no shared models.json mounted → none linked" 'test ! -e $HOME/.pi/agent/models.json && test ! -L $HOME/.pi/agent/models.json'
+exec_test "link-pi-models.sh links a mounted models.json" 'mkdir -p /tmp/lpm && echo "{}" > /tmp/lpm/models.json && PI_MODELS_JSON=/tmp/lpm/models.json PI_CODING_AGENT_DIR=/tmp/lpm/agent bash /usr/local/lib/pi-devbox/link-pi-models.sh >/dev/null && test "$(readlink /tmp/lpm/agent/models.json)" = /tmp/lpm/models.json && rm -rf /tmp/lpm'
 exec_test "pi-devbox-environment skill linked"     'test -L $HOME/.agents/skills/pi-devbox-environment && test -f $HOME/.agents/skills/pi-devbox-environment/SKILL.md && echo ok'
 exec_test "pi-extensions skill linked (fallback)"  'test -L $HOME/.agents/skills/pi-extensions && test -f $HOME/.agents/skills/pi-extensions/SKILL.md && echo ok'
 exec_test "mempalace skill linked (fallback)"      'test -L $HOME/.agents/skills/mempalace && test -f $HOME/.agents/skills/mempalace/SKILL.md && echo ok'