feat: Add interactive graph visualization with Claude-powered exploration

Adds `codegraph visualize` command that launches a localhost web UI for
visually exploring code relationships. Users can ask natural language
questions like "how does authentication work?" and see the relevant code
flow rendered as an interactive graph.

Key components:
- Visualizer HTTP server (src/visualizer/server.ts) with REST API
- Single-page frontend with Cytoscape.js graph + highlight.js code preview
- Claude CLI integration for intelligent query interpretation
- Dark theme, right-click context menus, keyboard shortcuts
- Detail panel with source code, callers, callees, hierarchy

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -16,7 +16,7 @@
     "build": "tsc && npm run copy-assets",
     "postinstall": "node scripts/postinstall.js",
     "preuninstall": "node dist/bin/uninstall.js",
-    "copy-assets": "node -e \"const fs=require('fs');fs.mkdirSync('dist/db',{recursive:true});fs.copyFileSync('src/db/schema.sql','dist/db/schema.sql');fs.mkdirSync('dist/extraction/wasm',{recursive:true});fs.readdirSync('src/extraction/wasm').filter(f=>f.endsWith('.wasm')).forEach(f=>fs.copyFileSync('src/extraction/wasm/'+f,'dist/extraction/wasm/'+f))\"",
+    "copy-assets": "node -e \"const fs=require('fs');fs.mkdirSync('dist/db',{recursive:true});fs.copyFileSync('src/db/schema.sql','dist/db/schema.sql');fs.mkdirSync('dist/extraction/wasm',{recursive:true});fs.readdirSync('src/extraction/wasm').filter(f=>f.endsWith('.wasm')).forEach(f=>fs.copyFileSync('src/extraction/wasm/'+f,'dist/extraction/wasm/'+f));fs.mkdirSync('dist/visualizer/public',{recursive:true});fs.copyFileSync('src/visualizer/public/index.html','dist/visualizer/public/index.html')\"",
     "dev": "tsc --watch",
     "cli": "npm run build && node dist/bin/codegraph.js",
     "test": "vitest run",