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/recreate-sanity-check.sh

@@ -277,6 +277,24 @@ else
   fail "~/.pi/agent/AGENTS.md missing or not a symlink"
 fi
 
+# shared models.json (link-pi-models.sh) — checked only when a shared file is
+# actually mounted, so a device without one is not judged by it.
+_mj_src="${PI_MODELS_JSON-/opt/fleet-pi/models.json}"
+case "$_mj_src" in "" | off | OFF | Off) _mj_src="" ;; esac
+if [ -n "$_mj_src" ] && [ -f "$_mj_src" ]; then
+  _mj="$HOME/.pi/agent/models.json"
+  if [ -L "$_mj" ] && [ "$(readlink "$_mj")" = "$_mj_src" ]; then
+    pass "~/.pi/agent/models.json -> $_mj_src (shared custom providers)"
+  elif [ -L "$_mj" ]; then
+    warn "~/.pi/agent/models.json links to $(readlink "$_mj"), not the mounted $_mj_src"
+  elif [ -e "$_mj" ]; then
+    warn "~/.pi/agent/models.json is a real file, so the mounted $_mj_src is NOT in use"
+  else
+    fail "$_mj_src is mounted but ~/.pi/agent/models.json is missing (image without link-pi-models.sh? run: ln -s $_mj_src ~/.pi/agent/models.json)"
+  fi
+fi
+unset _mj _mj_src
+
 # extensions deployed (pi-extensions) — expect ≥4 *.ts
 EXT_COUNT=$(ls -1 "$HOME"/.pi/agent/extensions/*.ts 2>/dev/null | wc -l | tr -d ' ')
 if [ "$EXT_COUNT" -ge 4 ]; then