Add community node protocol spec, Agent Skills, and make the failing tests hermetic

Community nodes (Phase 1 of plan.md): the protocol specification in
services/nodes/spec.py as the single source of truth, GET /nodes/spec.json,
a generated protocol docs page, the Community nodes API docs group, and the
implementation plan plus the residential proxy research. The no-open-proxy rule
is enforced in the spec (open_proxy cannot be set).

Agent Skills: register the Skills API docs group, cascade skills on account
deletion, add the nested CLAUDE.md, README and routes docs, and cover the
parser, the fetcher (end to end against a mock origin), the data layer and the
routes with 73 unit and 16 api tests.

Tests made hermetic with the repo's existing patterns instead of ambient
environment: embeddings tests set OPENROUTER_API_KEY via monkeypatch, the
workspace tests install FakeBackend via runtime.set_backend, and the skills
install tests use run_async. DockerCliBackend now returns 127 (command not
found) when the CLI is missing instead of raising FileNotFoundError, so a host
without docker answers with a clean error rather than a 500. The Providers tab
empty state no longer names the Model routes tab.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/api/uploads/generateimage.py

@@ -0,0 +1,34 @@
+# retoor <retoor@molodetz.nl>
+
+import requests
+from tests.api.uploads.uploadurl import _key, _upload_settings  # noqa: F401
+from tests.conftest import BASE_URL
+
+
+def test_generate_image_requires_auth():
+    response = requests.post(f"{BASE_URL}/uploads/generate-image", data={"prompt": "a cat"})
+    assert response.status_code == 401
+
+
+def test_generate_image_rejects_empty_prompt():
+    response = requests.post(
+        f"{BASE_URL}/uploads/generate-image",
+        headers={"X-API-KEY": _key()},
+        data={"prompt": ""},
+        allow_redirects=False,
+    )
+    assert response.status_code != 201
+
+
+def test_generate_image_upstream_failure_is_a_clean_json_error():
+    response = requests.post(
+        f"{BASE_URL}/uploads/generate-image",
+        headers={"X-API-KEY": _key()},
+        data={"prompt": "a lighthouse at dusk"},
+    )
+    assert response.status_code in (201, 502)
+    body = response.json()
+    if response.status_code == 502:
+        assert body["error"]
+    else:
+        assert body["uid"]