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.

.env.example

@@ -220,3 +220,16 @@ WANDB_API_KEY=
 # RL API Server URL (default: http://localhost:8080)
 # Change if running the rl-server on a different host/port
 # RL_API_URL=http://localhost:8080
+
+# =============================================================================
+# SKILLS HUB (GitHub integration for skill search/install/publish)
+# =============================================================================
+
+# GitHub Personal Access Token — for higher API rate limits on skill search/install
+# Get at: https://github.com/settings/tokens (Fine-grained recommended)
+# GITHUB_TOKEN=ghp_xxxxxxxxxxxxxxxxxxxx
+
+# GitHub App credentials (optional — for bot identity on PRs)
+# GITHUB_APP_ID=
+# GITHUB_APP_PRIVATE_KEY_PATH=
+# GITHUB_APP_INSTALLATION_ID=