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

optional-skills/DESCRIPTION.md

@@ -0,0 +1,22 @@
+# Optional Skills
+
+Official skills maintained by Nous Research that are **not activated by default**.
+
+These skills ship with the hermes-agent repository but are not copied to
+`~/.hermes/skills/` during setup. They are discoverable via the Skills Hub:
+
+```bash
+hermes skills search <query>     # finds optional skills labeled "official"
+hermes skills install <identifier>  # copies to ~/.hermes/skills/ and activates
+```
+
+## Why optional?
+
+Some skills are useful but not broadly needed by every user:
+
+- **Niche integrations** — specific paid services, specialized tools
+- **Experimental features** — promising but not yet proven
+- **Heavyweight dependencies** — require significant setup (API keys, installs)
+
+By keeping them optional, we keep the default skill set lean while still
+providing curated, tested, official skills for users who want them.