Add Skills Hub — universal skill search, install, and management from online registries

Implements the Hermes Skills Hub with agentskills.io spec compliance,
multi-registry skill discovery, security scanning, and user-driven
management via CLI and /skills slash command.

Core features:
- Security scanner (tools/skills_guard.py): 120 threat patterns across
  12 categories, trust-aware install policy (builtin/trusted/community),
  structural checks, unicode injection detection, LLM audit pass
- Hub client (tools/skills_hub.py): GitHub, ClawHub, Claude Code
  marketplace, and LobeHub source adapters with shared GitHubAuth
  (PAT + gh CLI + GitHub App), lock file provenance tracking, quarantine
  flow, and unified search across all sources
- CLI interface (hermes_cli/skills_hub.py): search, install, inspect,
  list, audit, uninstall, publish (GitHub PR), snapshot export/import,
  and tap management — powers both `hermes skills` and `/skills`

Spec conformance (Phase 0):
- Upgraded frontmatter parser to yaml.safe_load with fallback
- Migrated 39 SKILL.md files: tags/related_skills to metadata.hermes.*
- Added assets/ directory support and compatibility/metadata fields
- Excluded .hub/ from skill discovery in skills_tool.py

Updated 13 config/doc files including README, AGENTS.md, .env.example,
setup wizard, doctor, status, pyproject.toml, and docs.

hermes_cli/status.py

@@ -77,6 +77,7 @@ def show_status(args):
         "Tinker": "TINKER_API_KEY",
         "WandB": "WANDB_API_KEY",
         "ElevenLabs": "ELEVENLABS_API_KEY",
+        "GitHub": "GITHUB_TOKEN",
     }
     
     for name, env_var in keys.items():