Add comprehensive Mermaid.js diagram skills

Create new diagrams category with 8 specialized skills covering all major
Mermaid diagram types for visual documentation:

Core diagrams:
- mermaid-flowcharts: Process flows, decision trees, subgraphs
- mermaid-sequence-diagrams: Interactions, message flows, lifelines
- mermaid-class-state-diagrams: UML classes, state machines
- mermaid-er-diagrams: Database schemas, entity relationships

Advanced diagrams:
- mermaid-architecture-diagrams: C4, block diagrams, system design
- mermaid-charts: Pie, XY, quadrant, radar charts
- mermaid-project-diagrams: Gantt charts, timelines
- mermaid-specialized: Git graphs, Sankey, mindmaps

Each skill:
- ~320-450 lines optimized for context efficiency
- Comprehensive syntax coverage with practical examples
- Common patterns for real-world use cases
- Best practices and anti-patterns
- Integration tips with other diagram types

Also includes:
- diagrams/INDEX.md: Category overview and quick reference
- discover-diagrams gateway: Auto-discovery for visualization work

Skills support documentation workflows for APIs, architecture, databases,
project planning, and version control visualization.

Author: rand

skills/diagrams/INDEX.md

@@ -0,0 +1,151 @@
+# Diagrams Skills
+
+Visual documentation and diagramming with Mermaid.js and other tools.
+
+## Available Skills
+
+### Mermaid.js Core Diagrams
+
+**mermaid-flowcharts.md**
+- Process flow visualization
+- Node shapes and connections
+- Decision trees and branching
+- Subgraphs and organization
+- Common patterns: API flows, data pipelines, authentication
+
+**mermaid-sequence-diagrams.md**
+- Interaction and message flows
+- Participants and actors
+- Message arrow types
+- Activation boxes and control flow
+- Common patterns: REST API, microservices, retry logic
+
+**mermaid-class-state-diagrams.md**
+- UML class diagrams
+- Object relationships and hierarchies
+- State machines and transitions
+- Composite states and choice nodes
+- Common patterns: API models, repositories, workflows
+
+**mermaid-er-diagrams.md**
+- Entity-relationship diagrams
+- Database schema design
+- Cardinality and relationships
+- Keys and attributes
+- Common patterns: E-commerce, auth systems, CMS
+
+### Mermaid.js Advanced Diagrams
+
+**mermaid-architecture-diagrams.md**
+- C4 diagrams (Context, Container, Component)
+- Block diagrams
+- System architecture visualization
+- Infrastructure and deployment
+- Common patterns: Microservices, serverless, Kubernetes
+
+**mermaid-charts.md**
+- Data visualization charts
+- Pie charts for distributions
+- XY charts (line and bar)
+- Quadrant charts for prioritization
+- Radar charts for multi-dimensional comparison
+
+**mermaid-project-diagrams.md**
+- Gantt charts for project timelines
+- Task dependencies and critical path
+- Timeline diagrams for chronology
+- Milestone tracking
+- Common patterns: Release plans, sprints, migrations
+
+**mermaid-specialized.md**
+- Git graphs for version control workflows
+- Sankey diagrams for data flows
+- Mindmaps for concept hierarchies
+- Requirement diagrams
+- Common patterns: Gitflow, budget allocation, learning paths
+
+## Quick Reference
+
+### When to Use Which Diagram
+
+**Process & Logic**:
+- Flowchart: Algorithm, decision tree, workflow
+- Sequence: API interaction, protocol, communication
+
+**Structure & Design**:
+- Class: Object model, API structure, OOP design
+- ER: Database schema, data relationships
+- Architecture: System design, infrastructure, deployment
+
+**Data & Metrics**:
+- Pie: Distribution, percentages, breakdown
+- XY: Trends, comparisons, time series
+- Quadrant: Prioritization, risk assessment
+- Radar: Skill assessment, multi-dimensional comparison
+- Sankey: Flow analysis, conversion funnels
+
+**Planning & History**:
+- Gantt: Project schedule, dependencies, critical path
+- Timeline: Product roadmap, history, milestones
+- GitGraph: Branching strategy, release workflow
+
+**Ideation & Organization**:
+- Mindmap: Brainstorming, concept breakdown, learning paths
+- State: Lifecycle, protocol states, FSM
+
+## Usage Patterns
+
+### Documentation
+```bash
+# In markdown files
+```mermaid
+graph TD
+    A --> B
+```
+```
+
+### Live Preview
+- Use https://mermaid.live for development
+- Auto-renders in GitHub/GitLab markdown
+- VS Code: Install "Mermaid Preview" extension
+
+### Combining Diagrams
+
+**Start broad, drill down**:
+1. Architecture diagram → System context
+2. Sequence diagram → Runtime behavior
+3. Class/ER diagram → Data/object structure
+4. Flowchart → Business logic detail
+
+**Example workflow**:
+- C4 Context → What systems exist?
+- C4 Container → What technologies?
+- Sequence → How do they interact?
+- Class → What's the data model?
+- Flowchart → What's the algorithm?
+
+## Best Practices
+
+1. **One diagram, one purpose**: Don't mix abstraction levels
+2. **Label everything**: Axes, relationships, transitions
+3. **Limit complexity**: Split large diagrams into focused views
+4. **Use consistent styling**: Establish patterns for colors, shapes
+5. **Document assumptions**: Add notes for non-obvious details
+6. **Version control**: Track diagram changes like code
+
+## Integration
+
+These skills integrate with:
+- **api-*** skills: Document API design with sequence/class diagrams
+- **database-*** skills: Schema design with ER diagrams
+- **testing-*** skills: Test flows with sequence/flowcharts
+- **containers-*** skills: Deployment with architecture diagrams
+- **git-*** skills: Workflow visualization with Git graphs
+
+## Resources
+
+- Mermaid.js: https://mermaid.js.org
+- Live Editor: https://mermaid.live
+- GitHub Docs: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams
+- C4 Model: https://c4model.com
+- UML Reference: https://www.uml-diagrams.org