feat: add mermaid diagram ASCII rendering support

[tawago]

aiming to close #342

Summary

Adds support for rendering mermaid code blocks as ASCII art diagrams, addressing #342.

• Integrates mermaid-ascii library for Go-native rendering
• Supports flowcharts (graph LR/TD/RL/BT) and sequence diagrams
• Graceful fallback: preserves original code block when diagram type or feature is unsupported
• Single --mermaid flag (enabled by default, use --mermaid=false to disable)
• Works in both CLI and TUI modes

Example

graph LR
    A[Start] --> B{Decision}
    B -->|Yes| C[OK]
    B -->|No| D[Cancel]

Loading

Renders as:

┌───────┐     ┌──────────┐
│ Start │────▶│ Decision │
└───────┘     └────┬─────┘
                   │
        ┌──────────┴──────────┐
        ▼                     ▼
   ┌────────┐            ┌─────────┐
   │   OK   │            │ Cancel  │
   └────────┘            └─────────┘

Unsupported Features (graceful fallback)

The following are detected and fall back to showing the original code block:

• Diagram types: pie, gantt, classDiagram, stateDiagram, erDiagram, etc.
• Sequence features: loop, alt, opt, par, Note, activate/deactivate

Test plan

• Unit tests for mermaid package (10 test cases)
• go test ./... passes
• go vet ./... passes