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.

cli-config.yaml.example

@@ -244,6 +244,7 @@ platform_toolsets:
 #   vision       - vision_analyze  (requires OPENROUTER_API_KEY)
 #   image_gen    - image_generate  (requires FAL_KEY)
 #   skills       - skills_list, skill_view
+#   skills_hub   - skill_hub (search/install/manage from online registries — user-driven only)
 #   moa          - mixture_of_agents  (requires OPENROUTER_API_KEY)
 #   todo         - todo (in-memory task planning, no deps)
 #   tts          - text_to_speech  (Edge TTS free, or ELEVENLABS/OPENAI key)