feat: add uninstall command to remove OpenCode and clean up related files

[kunaljaykam]

Fix for Issue #3900

there's one more pr on the same issue but it didn't looked right: #3904

Files created/modified:

1. src/cli/cmd/uninstall.ts - New uninstall command implementation
2. src/index.ts - Registered the UninstallCommand
3. README.md - Added uninstall documentation section

Key features:

• ✅ Confirmation dialog before uninstalling
• ✅ Detects installation method (curl, npm, pnpm, bun, brew)
• ✅ Handles curl installs by checking common binary locations
• ✅ Option to keep user data with --keep-data
• ✅ Friendly error messages and progress indicators
• ✅ Thank you message with feedback link