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>

src/bin/codegraph.ts

@@ -955,6 +955,70 @@ program
     }
   });
 
+/**
+ * codegraph visualize [path]
+ */
+program
+  .command('visualize [path]')
+  .description('Open interactive graph visualization in your browser')
+  .option('-p, --port <port>', 'Port to listen on (default: auto)', parseInt)
+  .option('--no-open', 'Do not open browser automatically')
+  .action(async (pathArg: string | undefined, options: { port?: number; open?: boolean }) => {
+    const projectPath = resolveProjectPath(pathArg);
+
+    try {
+      if (!isInitialized(projectPath)) {
+        error(`CodeGraph not initialized in ${projectPath}`);
+        info('Run "codegraph init -i" first');
+        process.exit(1);
+      }
+
+      const { default: CodeGraph } = await loadCodeGraph();
+      const cg = await CodeGraph.open(projectPath);
+      const stats = cg.getStats();
+
+      console.log(chalk.bold('\n  CodeGraph Explorer\n'));
+      info(`Project: ${projectPath}`);
+      info(`Indexed: ${formatNumber(stats.nodeCount)} nodes, ${formatNumber(stats.edgeCount)} edges, ${formatNumber(stats.fileCount)} files\n`);
+
+      const { VisualizerServer } = await import('../visualizer/server');
+      const server = new VisualizerServer(cg);
+      const { url } = await server.start({ port: options.port, openBrowser: options.open !== false });
+
+      success(`Visualizer running at ${chalk.cyan(url)}`);
+      console.log(chalk.dim('  Press Ctrl+C to stop\n'));
+
+      // Open browser
+      if (options.open !== false) {
+        const openCmd = process.platform === 'darwin' ? 'open' :
+                        process.platform === 'win32' ? 'start' : 'xdg-open';
+        spawn(openCmd, [url], { detached: true, stdio: 'ignore' }).unref();
+      }
+
+      // Handle shutdown — force exit on second Ctrl+C
+      let shuttingDown = false;
+      const shutdown = () => {
+        if (shuttingDown) {
+          process.exit(1);
+        }
+        shuttingDown = true;
+        console.log(chalk.dim('\n  Shutting down...'));
+        server.stop().then(() => {
+          cg.close();
+          process.exit(0);
+        }).catch(() => process.exit(1));
+        // Force exit after 2s if graceful shutdown hangs
+        setTimeout(() => process.exit(1), 2000).unref();
+      };
+      process.on('SIGINT', shutdown);
+      process.on('SIGTERM', shutdown);
+    } catch (err) {
+      captureException(err);
+      error(`Failed to start visualizer: ${err instanceof Error ? err.message : String(err)}`);
+      process.exit(1);
+    }
+  });
+
 /**
  * codegraph mark-dirty [path]
  *