Add interactive code graph visualization

[xy200303]

Summary

• Add codegraph visualize [symbol] for generating an interactive HTML visualization of indexed code relationships.
• Implement a custom animated Canvas force graph instead of using a generic graph visualization dependency.
• Add force controls for gravity, repulsion, and link distance so users can tune aggregation and spread directly in the browser.
• Add graph layout actions for fit, spread, cluster, expand, pause/resume animation, and reset.
• Update CLI documentation and add focused visualization tests.

Changes

• Added src/visualization.ts with a self-contained HTML graph renderer.
• Added the visualize command in src/bin/codegraph.ts.
• Added runtime edge kind support for visualization filtering and rendering.
• Added node and edge filters with color swatches.
• Added adjustable graph physics:
  • Gravity controls center attraction and aggregation.
  • Repulsion controls node separation.
  • Link distance controls edge spring length.
• Added focused test coverage in __tests__/visualization.test.ts.
• Updated documentation in README.md and site/src/content/docs/reference/cli.md.

Design Note

• The visualization should reserve a dedicated legend area in the left control panel.
• The legend area can sit near the existing node and edge filters, so users can quickly understand color meaning without covering the Canvas.
• The legend should support both node kinds and edge kinds, and can share the same color mapping used by the renderer.

Verification

• npm run build
• npx vitest run __tests__/visualization.test.ts

Notes

• No third-party graph rendering dependency was added.
• No project PR template was found, so this PR uses the default English format.
• Branch: feature/graph-visualization