TUI: clickable hyperlinks via OSC 8 escape sequences

[bug-ops]

Summary

Add clickable hyperlink support in TUI chat view using OSC 8 terminal escape sequences. URLs in model responses, file paths in tool output, and issue/PR references would become clickable.

Background

OSC 8 is widely supported by modern terminals: iTerm2, WezTerm, Kitty, Ghostty, Windows Terminal, GNOME Terminal. The escape sequence format is \033]8;;URL\033\\visible text\033]8;;\033\\.

Ratatui does not yet have native OSC 8 support (ratatui/ratatui#1227, status: Design Needed), so implementation would require either raw escape sequence output via crossterm or the osc8 crate.

Use Cases

1. URLs in model responses — http/https links become clickable
2. File paths in tool output — open in editor via file:// scheme
3. GitHub references — #123 links to issue/PR URL

Implementation Notes

• Detect URLs in rendered text spans using regex
• Wrap matched spans with OSC 8 open/close sequences before writing to terminal
• Graceful degradation: terminals without OSC 8 support simply ignore the sequences
• Consider feature-gating if the osc8 crate adds weight

References

• OSC 8 spec
• Terminal adoption list
• ratatui tracking issue
• osc8 crate