feat(#23): implement mosaic-plugin-brain skill

- Add brain skill for Ideas/Brain API integration
- Quick capture for brain dumps
- Semantic search and query capabilities
- Full CRUD operations on ideas
- Tag management and filtering
- Shell script CLI with colored output
- Comprehensive documentation (SKILL.md, README.md)
- Configuration via env vars or ~/.config/mosaic/brain.conf

packages/skills/brain/README.md

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+# Mosaic Brain Skill
+
+A Clawdbot skill for integrating with Mosaic Stack's Ideas/Brain API.
+
+## Features
+
+- **Quick Brain Dump**: Capture ideas rapidly with `capture` command
+- **Semantic Search**: Query your knowledge base using natural language
+- **Full CRUD**: Create, read, update, delete ideas with rich metadata
+- **Tag Management**: Organize ideas with tags
+- **Flexible Queries**: Filter by status, tags, dates, and more
+
+## Installation
+
+1. Copy this skill to your Clawdbot skills directory or link it:
+   ```bash
+   ln -s ~/src/mosaic-stack/packages/skills/brain ~/.config/clawdbot/skills/mosaic-brain
+   ```
+
+2. Configure your Mosaic Stack connection:
+   ```bash
+   mkdir -p ~/.config/mosaic
+   cat > ~/.config/mosaic/brain.conf <<EOF
+   MOSAIC_API_URL=http://localhost:3001
+   MOSAIC_WORKSPACE_ID=your-workspace-uuid
+   MOSAIC_API_TOKEN=your-auth-token
+   EOF
+   ```
+
+3. Verify setup:
+   ```bash
+   ./brain.sh list --limit 5
+   ```
+
+## Usage
+
+See [SKILL.md](./SKILL.md) for detailed usage documentation.
+
+### Quick Examples
+
+```bash
+# Capture a quick thought
+./brain.sh capture "Need to implement user authentication for the new app"
+
+# Create detailed idea
+./brain.sh create \
+  --title "Authentication System" \
+  --content "Implement OAuth2 + JWT auth..." \
+  --tags "backend,security,auth" \
+  --priority "HIGH"
+
+# Search semantically
+./brain.sh query "What did I say about authentication?"
+
+# List recent ideas
+./brain.sh list --limit 10
+
+# Update an idea
+./brain.sh update <id> --status "IN_PROGRESS" --add-tags "urgent"
+```
+
+## API Reference
+
+This skill interfaces with the following Mosaic Stack endpoints:
+
+- `POST /api/ideas/capture` - Quick capture
+- `POST /api/ideas` - Create full idea
+- `GET /api/ideas` - List ideas
+- `GET /api/ideas/:id` - Get idea
+- `PATCH /api/ideas/:id` - Update idea
+- `DELETE /api/ideas/:id` - Delete idea
+- `POST /api/brain/query` - Semantic query
+- `GET /api/brain/search` - Keyword search
+
+## Development
+
+### Requirements
+
+- `curl` - HTTP client
+- `jq` - JSON processor
+- Mosaic Stack API running and accessible
+
+### Testing
+
+```bash
+# Test capture
+./brain.sh capture "Test idea $(date)"
+
+# Test search
+./brain.sh search "test"
+
+# Test query
+./brain.sh query "recent test ideas"
+```
+
+## License
+
+Part of the Mosaic Stack ecosystem.