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.

optional-skills/autonomous-ai-agents/DESCRIPTION.md

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+Optional autonomous AI agent integrations — external coding agent CLIs
+that can be delegated to for independent coding tasks.

optional-skills/autonomous-ai-agents/blackbox/SKILL.md

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+---
+name: blackbox
+description: Delegate coding tasks to Blackbox AI CLI agent. Multi-model agent with built-in judge that runs tasks through multiple LLMs and picks the best result. Requires the blackbox CLI and a Blackbox AI API key.
+version: 1.0.0
+author: Hermes Agent (Nous Research)
+license: MIT
+metadata:
+  hermes:
+    tags: [Coding-Agent, Blackbox, Multi-Agent, Judge, Multi-Model]
+    related_skills: [claude-code, codex, hermes-agent]
+---
+
+# Blackbox CLI
+
+Delegate coding tasks to [Blackbox AI](https://www.blackbox.ai/) via the Hermes terminal. Blackbox is a multi-model coding agent CLI that dispatches tasks to multiple LLMs (Claude, Codex, Gemini, Blackbox Pro) and uses a judge to select the best implementation.
+
+The CLI is [open-source](https://github.com/blackboxaicode/cli) (GPL-3.0, TypeScript, forked from Gemini CLI) and supports interactive sessions, non-interactive one-shots, checkpointing, MCP, and vision model switching.
+
+## Prerequisites
+
+- Node.js 20+ installed
+- Blackbox CLI installed: `npm install -g @blackboxai/cli`
+- Or install from source:
+  ```
+  git clone https://github.com/blackboxaicode/cli.git
+  cd cli && npm install && npm install -g .
+  ```
+- API key from [app.blackbox.ai/dashboard](https://app.blackbox.ai/dashboard)
+- Configured: run `blackbox configure` and enter your API key
+- Use `pty=true` in terminal calls — Blackbox CLI is an interactive terminal app
+
+## One-Shot Tasks
+
+```
+terminal(command="blackbox --prompt 'Add JWT authentication with refresh tokens to the Express API'", workdir="/path/to/project", pty=true)
+```
+
+For quick scratch work:
+```
+terminal(command="cd $(mktemp -d) && git init && blackbox --prompt 'Build a REST API for todos with SQLite'", pty=true)
+```
+
+## Background Mode (Long Tasks)
+
+For tasks that take minutes, use background mode so you can monitor progress:
+
+```
+# Start in background with PTY
+terminal(command="blackbox --prompt 'Refactor the auth module to use OAuth 2.0'", workdir="~/project", background=true, pty=true)
+# Returns session_id
+
+# Monitor progress
+process(action="poll", session_id="<id>")
+process(action="log", session_id="<id>")
+
+# Send input if Blackbox asks a question
+process(action="submit", session_id="<id>", data="yes")
+
+# Kill if needed
+process(action="kill", session_id="<id>")
+```
+
+## Checkpoints & Resume
+
+Blackbox CLI has built-in checkpoint support for pausing and resuming tasks:
+
+```
+# After a task completes, Blackbox shows a checkpoint tag
+# Resume with a follow-up task:
+terminal(command="blackbox --resume-checkpoint 'task-abc123-2026-03-06' --prompt 'Now add rate limiting to the endpoints'", workdir="~/project", pty=true)
+```
+
+## Session Commands
+
+During an interactive session, use these commands:
+
+| Command | Effect |
+|---------|--------|
+| `/compress` | Shrink conversation history to save tokens |
+| `/clear` | Wipe history and start fresh |
+| `/stats` | View current token usage |
+| `Ctrl+C` | Cancel current operation |
+
+## PR Reviews
+
+Clone to a temp directory to avoid modifying the working tree:
+
+```
+terminal(command="REVIEW=$(mktemp -d) && git clone https://github.com/user/repo.git $REVIEW && cd $REVIEW && gh pr checkout 42 && blackbox --prompt 'Review this PR against main. Check for bugs, security issues, and code quality.'", pty=true)
+```
+
+## Parallel Work
+
+Spawn multiple Blackbox instances for independent tasks:
+
+```
+terminal(command="blackbox --prompt 'Fix the login bug'", workdir="/tmp/issue-1", background=true, pty=true)
+terminal(command="blackbox --prompt 'Add unit tests for auth'", workdir="/tmp/issue-2", background=true, pty=true)
+
+# Monitor all
+process(action="list")
+```
+
+## Multi-Model Mode
+
+Blackbox's unique feature is running the same task through multiple models and judging the results. Configure which models to use via `blackbox configure` — select multiple providers to enable the Chairman/judge workflow where the CLI evaluates outputs from different models and picks the best one.
+
+## Key Flags
+
+| Flag | Effect |
+|------|--------|
+| `--prompt "task"` | Non-interactive one-shot execution |
+| `--resume-checkpoint "tag"` | Resume from a saved checkpoint |
+| `--yolo` | Auto-approve all actions and model switches |
+| `blackbox session` | Start interactive chat session |
+| `blackbox configure` | Change settings, providers, models |
+| `blackbox info` | Display system information |
+
+## Vision Support
+
+Blackbox automatically detects images in input and can switch to multimodal analysis. VLM modes:
+- `"once"` — Switch model for current query only
+- `"session"` — Switch for entire session
+- `"persist"` — Stay on current model (no switch)
+
+## Token Limits
+
+Control token usage via `.blackboxcli/settings.json`:
+```json
+{
+  "sessionTokenLimit": 32000
+}
+```
+
+## Rules
+
+1. **Always use `pty=true`** — Blackbox CLI is an interactive terminal app and will hang without a PTY
+2. **Use `workdir`** — keep the agent focused on the right directory
+3. **Background for long tasks** — use `background=true` and monitor with `process` tool
+4. **Don't interfere** — monitor with `poll`/`log`, don't kill sessions because they're slow
+5. **Report results** — after completion, check what changed and summarize for the user
+6. **Credits cost money** — Blackbox uses a credit-based system; multi-model mode consumes credits faster
+7. **Check prerequisites** — verify `blackbox` CLI is installed before attempting delegation