feat: improve file watcher with chokidar and better ignore patterns

[thdxr]

Summary

• Replace native fs.watch with chokidar for more reliable file watching
• Add comprehensive ignore patterns for common directories and files
• Make file watcher opt-in with OPENCODE_EXPERIMENTAL_WATCHER flag
• Add watcher configuration option for custom ignore patterns
• Improve file watcher initialization timing

Changes

• packages/opencode/src/file/watcher.ts: New chokidar-based implementation
• packages/opencode/src/file/ignore.ts: New ignore patterns system
• packages/opencode/package.json: Add chokidar dependency
• packages/opencode/src/config/config.ts: Add watcher configuration schema
• packages/opencode/src/flag/flag.ts: Replace disable flag with experimental flag