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/e2e/openai/v1/embeddings.py

@@ -49,6 +49,7 @@ def test_embeddings_auth_passes_upstream_unreachable(alice):
         page,
         gateway_embed_enabled="1",
         gateway_embed_url="http://127.0.0.1:9/embeddings",
+        gateway_embed_key="test-embed-key",
         gateway_access_key="embedkey",
     )
     try:
@@ -68,7 +69,7 @@ def test_embeddings_auth_passes_upstream_unreachable(alice):
         )
         assert with_admin.status_code == 502
     finally:
-        _config(page, gateway_access_key="")
+        _config(page, gateway_access_key="", gateway_embed_key="")
         page.request.post(f"{BASE_URL}/admin/services/openai/start")