docs: Add TUI documentation and update roadmap

[paperdiamond]

Summary

• Added Interactive TUI Mode section with usage instructions
• Documented keyboard shortcuts for TUI navigation
• Added TUI feature to features list
• Marked TUI as completed in roadmap

Test plan

• Review README changes for accuracy
• Test that pnpm tui command works as documented

[paperdiamond]

Great docs update! A few suggestions:

1. We should add a screenshot or GIF showing the TUI in action - visual learners will appreciate it
2. Consider adding a note about required environment variables for the TUI (LINEAR_API_KEY, etc.)
3. The keyboard shortcuts section could mention Ctrl+C as an alternative to 'q' for quitting

/create-issues

[paperdiamond]

Also thinking about future improvements:

1. Add support for filtering comments by author - would help in large PRs
2. Consider adding a "dry run" mode that shows what issues would be created without actually creating them
3. Would be nice to have vim-style search (/) for quickly finding specific comments

Low priority but would be nice to have!