feat(skills): integrate NVIDIA/skills as a trusted skills hub tap

NVIDIA's verified skills catalog (https://github.com/NVIDIA/skills) ships
NVIDIA-signed skills for CUDA-X, AIQ, cuOpt, cuPyNumeric, DeepStream, NeMo,
NemoClaw and the Skill Card Generator — each bundle carrying a detached
`skill.oms.sig` signature, a governance `skill-card.md`, and `evals/`. The
sync pipeline drops any skill missing those artifacts before publishing.

Changes:
- tools/skills_hub.py: add NVIDIA/skills to GitHubSource.DEFAULT_TAPS so
  it lights up in `hermes skills browse`, `hermes skills search <q>`, the
  twice-daily skills-index build, and the docs-site Skills Hub page
  (https://hermes-agent.nousresearch.com/docs/skills) automatically.
- tools/skills_guard.py: add NVIDIA/skills to TRUSTED_REPOS so installs
  resolve to trust_level="trusted" (looser install policy than community).
- website/scripts/extract-skills.py: map the `github` source id to a
  friendly "NVIDIA" pill label for the docs hub page.
- website/src/pages/skills/index.tsx: register the NVIDIA pill (green
  #76b900) and slot it into SOURCE_ORDER after HuggingFace.
- website/docs/user-guide/features/skills.md (+ zh-Hans i18n): document
  the new default tap and the expanded trusted-repos list.
- tests/tools/test_skills_guard.py: assert NVIDIA/skills resolves to
  "trusted" (including the skills-sh-wrapped form).
- tests/tools/test_skills_hub.py: invariant — every TRUSTED_REPOS entry
  must be reachable via GitHubSource.DEFAULT_TAPS (prevents future
  trusted repos from being declared but never browseable).

Validation:
- Live GitHub fetch: `src.fetch('NVIDIA/skills/skills/aiq-deploy')` pulled
  17 files including SKILL.md (13 KB), skill-card.md, skill.oms.sig, and
  the full references/ + evals/ tree. trust_level="trusted".
- Live inspect resolved name, description, and trust correctly.
- All 193 existing skills_guard + skills_hub tests still pass.

website/scripts/extract-skills.py

@@ -95,6 +95,7 @@ GITHUB_TAP_LABELS = {
     "openai/skills": "OpenAI",
     "anthropics/skills": "Anthropic",
     "huggingface/skills": "HuggingFace",
+    "NVIDIA/skills": "NVIDIA",
     "VoltAgent/awesome-agent-skills": "VoltAgent",
     "garrytan/gstack": "gstack",
     "MiniMax-AI/cli": "MiniMax",