fix(#539): expose Fd/Write/Close on csiuReader for raw mode

[oryaacov]

Summary

• Root cause: csiuReader (introduced in 2afee24 / Shift+letter keys silently dropped — CSI u reader not wired into input pipeline #535) only implements io.Reader. Bubble Tea checks for the term.File interface (Fd() uintptr, Write(), Close()) to detect a terminal and enable raw mode via tcsetattr. Without raw mode, the terminal stays in cooked mode and echoes keystrokes as text instead of routing them to the TUI.
• Fix: Delegate Fd(), Write(), and Close() to the underlying *os.File when present. This preserves the CSI u translation while allowing Bubble Tea to set raw mode normally.
• This is a better fix than removing CSIuReader entirely (as proposed in fix: remove CSIuReader from tea.WithInput — breaks terminal raw mode #538), because it keeps the Kitty keyboard protocol translation for terminals that ignore DisableKittyKeyboard.

Fixes #539, #544

Test plan

• Existing TestCSIuReader* and TestParseCSIu* tests pass (23 tests)
• Launch agent-deck in a terminal — arrow keys, j/k, mouse scroll all work
• Launch in Ghostty/Foot (Kitty protocol terminals) — Shift+letter hotkeys work
• Quit the TUI — terminal returns to normal (no reset needed)

🤖 Generated with Claude Code

[asheshgoplani]

Thanks for this — exposing Fd/Write/Close on csiuReader is the architecturally cleaner approach if we want to keep the reader. For the hotfix we went with PR #541's revert route because users across iTerm2/Ghostty/WezTerm were blocked and the revert was the lowest-risk path.

If we decide to re-introduce CSI u translation later, this approach is a good starting point — worth pairing with the SGR mouse terminator whitelist fix from #541 and explicit term.MakeRaw fallback. Feel free to reopen against main if you want to land this as a proper fix for a future release.

Closing for now. 🙏