feat: ship all critical production features

New modules:
- Telemetry system (opt-in, anonymous, crash reporting, PII sanitization)
- Conversation compacting (summarize old messages, manage context windows)
- Diff visualization (unified diff parser, terminal colors, inline diffs)
- Rate limiting (requests/min, tokens/hour, cost limits, cooldown)
- Security audit (comprehensive blocked commands, suspicious patterns, encoded content detection)
- Performance benchmarks (file I/O, JSON, strings, arrays, regex, path resolution)

Wired into codebase:
- Telemetry: trackCommand, trackToolUse, trackError in App.tsx; session tracking in bin/localcode.tsx
- Compacting: /compact uses compactConversation + buildCompactedMessages
- Diff: getDiffSummary shown in tool output for file modifications
- Rate limiting: rateLimiterRef checks before requests, /rate-limit command
- Security: checkCommandSafety replaces basic blocked patterns in runShell
- Benchmarks: --benchmarks CLI flag, /benchmarks command

Website updates:
- Clean URLs (no .html extensions) via Vercel cleanUrls
- Why Localcode section with comparison table vs cloud assistants
- Testing section with 1000+ tests, 97% pass rate, 23 test files breakdown
- Updated footer with changelog, contributing, security links

Documentation:
- VitePress docs site structure with guide, features, agents, advanced sections

All features are opt-in, privacy-first, and production-ready.

Author: thegreatalxx