fix(tui): improve macOS terminal restoration on exit

[RandyMcMillan]

fixes: #7861

Summary:

Fixes the "dead terminal" issue that occurs when opencode exits on macOS, leaving users with a corrupted terminal state that requires manual reset.

Changes:

• ✅ Enhanced signal handling for SIGINT, SIGHUP, SIGTERM
• ✅ Improved suspend/resume with proper alternate screen buffer management
• ✅ Comprehensive exit cleanup with terminal reset sequences
• ✅ Added macOS terminal detection utility
• ✅ Added test script for validation
• ✅ Integrated tests into CI workflows

Technical Details:

• Uses proper ANSI escape sequences for terminal restoration
• Implements graceful signal cleanup to prevent memory leaks
• Added platform-specific terminal buffer management
• All TypeScript errors resolved

Testing:

• ✅ Manual testing on macOS Terminal and iTerm2
• ✅ Automated test script validates all functionality
• ✅ CI integration ensures future compatibility