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/SKILL.md

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+---
+name: mosaic-brain
+description: Capture ideas, brain dumps, and semantic search across your Mosaic Stack knowledge base.
+homepage: https://mosaicstack.dev
+metadata: {"clawdbot":{"emoji":"🧠","requires":{"bins":["curl","jq"]}}}
+---
+
+# Mosaic Brain
+
+Capture and search ideas, brain dumps, and notes in your Mosaic Stack workspace. Provides quick capture, semantic search, and tag management.
+
+## Setup
+
+1. Ensure Mosaic Stack API is running (default: `http://localhost:3001`)
+2. Set environment variables:
+   ```bash
+   export MOSAIC_API_URL="http://localhost:3001"
+   export MOSAIC_WORKSPACE_ID="your-workspace-uuid"
+   export MOSAIC_API_TOKEN="your-auth-token"
+   ```
+
+3. Optionally create `~/.config/mosaic/brain.conf`:
+   ```bash
+   MOSAIC_API_URL=http://localhost:3001
+   MOSAIC_WORKSPACE_ID=your-workspace-uuid
+   MOSAIC_API_TOKEN=your-auth-token
+   ```
+
+## Common Commands
+
+### Quick Brain Dump
+Capture an idea quickly without thinking about categorization:
+```bash
+# Quick capture
+./brain.sh capture "Had a great idea about using AI for code reviews"
+
+# Capture with title
+./brain.sh capture "Remember to follow up with the team about the new feature" --title "Team Follow-up"
+```
+
+### Create Detailed Idea
+Create an idea with full metadata:
+```bash
+./brain.sh create \
+  --title "New Feature Idea" \
+  --content "Implement semantic search across all documents..." \
+  --tags "feature,ai,search" \
+  --category "product"
+```
+
+### Semantic Search
+Search your brain using natural language:
+```bash
+# Query your knowledge base
+./brain.sh query "What did I say about AI and code reviews?"
+
+# Search with limit
+./brain.sh search "project ideas" --limit 10
+```
+
+### List Ideas
+```bash
+# List recent ideas
+./brain.sh list
+
+# List with filters
+./brain.sh list --limit 20 --tags "work,urgent"
+```
+
+### Get Specific Idea
+```bash
+./brain.sh get <idea-id>
+```
+
+### Update Idea
+```bash
+./brain.sh update <idea-id> \
+  --title "Updated Title" \
+  --tags "new,tags" \
+  --status "IN_PROGRESS"
+```
+
+### Delete Idea
+```bash
+./brain.sh delete <idea-id>
+```
+
+### Tag Management
+```bash
+# List all tags
+./brain.sh tags
+
+# Add tags to an idea
+./brain.sh update <idea-id> --add-tags "urgent,review"
+
+# Remove tags from an idea
+./brain.sh update <idea-id> --remove-tags "old-tag"
+```
+
+## API Endpoints
+
+The skill uses these Mosaic Stack API endpoints:
+
+- **POST /api/ideas/capture** - Quick brain dump (minimal fields)
+- **POST /api/ideas** - Create full idea with metadata
+- **GET /api/ideas** - List ideas with filters
+- **GET /api/ideas/:id** - Get specific idea
+- **PATCH /api/ideas/:id** - Update idea
+- **DELETE /api/ideas/:id** - Delete idea
+- **POST /api/brain/query** - Semantic search/query
+- **GET /api/brain/search** - Search ideas
+
+## Configuration
+
+Config file priority:
+1. Environment variables
+2. `~/.config/mosaic/brain.conf`
+3. Default values
+
+Required settings:
+- `MOSAIC_API_URL` - API base URL (default: http://localhost:3001)
+- `MOSAIC_WORKSPACE_ID` - Your workspace UUID
+- `MOSAIC_API_TOKEN` - Authentication token
+
+## Usage from Clawdbot
+
+Natural language examples:
+- "Remember this..." / "Note to self..." → Quick capture
+- "What did I say about..." → Semantic search
+- "Show me my recent ideas" → List ideas
+- "Brain dump: [content]" → Quick capture
+
+## Notes
+
+- Quick capture (`capture`) is fastest - just dump your thought
+- Semantic search (`query`) uses AI to find relevant ideas
+- Regular search is keyword-based, faster but less intelligent
+- Tags are automatically indexed for quick filtering
+- Ideas support markdown formatting in content