feat: optional skills — official skills shipped but not activated by default

Add 'optional-skills/' directory for official skills that ship with the repo
but are not copied to ~/.hermes/skills/ during setup. They are:
- NOT shown to the model in the system prompt
- NOT copied during hermes setup/update
- Discoverable via 'hermes skills search' labeled as 'official'
- Installable via 'hermes skills install' with builtin trust (no third-party warning)
- Auto-categorized on install based on directory structure

Implementation:
- OptionalSkillSource adapter in tools/skills_hub.py (search/fetch/inspect)
- Added to create_source_router() as first source (highest priority)
- Trust level 'builtin' for official skills in skills_guard.py
- Friendly install message for official skills (no third-party warning)
- 'official' label in cyan in search results and skill list

First optional skill: Blackbox CLI (autonomous-ai-agents/blackbox)
- Multi-model coding agent with built-in judge/Chairman pattern
- Delegates to Claude, Codex, Gemini, and Blackbox models
- Open-source CLI (GPL-3.0, TypeScript, forked from Gemini CLI)
- Requires paid Blackbox AI API key

Refs: #475

tools/skills_guard.py

@@ -1046,6 +1046,9 @@ def _get_configured_model() -> str:
 
 def _resolve_trust_level(source: str) -> str:
     """Map a source identifier to a trust level."""
+    # Official optional skills shipped with the repo
+    if source.startswith("official/") or source == "official":
+        return "builtin"
     # Check if source matches any trusted repo
     for trusted in TRUSTED_REPOS:
         if source.startswith(trusted) or source == trusted: