fix(tui): add modifyOtherKeys fallback for modified enter keys in tmux

When Kitty keyboard protocol is not available (e.g. inside tmux), fall
back to xterm modifyOtherKeys mode 2 so that Shift+Enter, Ctrl+Enter,
and other modified keys are distinguishable from plain Enter.

tmux users need to add to ~/.tmux.conf:
  set -g extended-keys on
  set -g extended-keys-format csi-u

closes #1872

packages/coding-agent/docs/tmux.md

@@ -0,0 +1,39 @@
+# tmux Setup
+
+Pi works inside tmux, but tmux strips modifier information from certain keys by default. Without configuration, `Shift+Enter` and `Ctrl+Enter` are indistinguishable from plain `Enter`.
+
+## Required Configuration
+
+Add to `~/.tmux.conf`:
+
+```tmux
+set -g extended-keys on
+set -g extended-keys-format csi-u
+```
+
+Then restart tmux (not just reload):
+
+```bash
+tmux kill-server
+tmux
+```
+
+This tells tmux to forward modified key sequences in CSI-u format when an application requests extended key reporting. Pi requests this automatically when Kitty keyboard protocol is not available.
+
+## What This Fixes
+
+Without this config, tmux collapses modified enter keys to plain `\r`:
+
+| Key | Without config | With config |
+|-----|---------------|-------------|
+| Enter | `\r` | `\r` |
+| Shift+Enter | `\r` | `\x1b[13;2u` |
+| Ctrl+Enter | `\r` | `\x1b[13;5u` |
+| Alt/Option+Enter | `\x1b\r` | `\x1b[13;3u` |
+
+This affects the default keybindings (`Enter` to submit, `Shift+Enter` for newline) and any custom keybindings using modified enter keys.
+
+## Requirements
+
+- tmux 3.2 or later (run `tmux -V` to check)
+- A terminal emulator that supports extended keys (Ghostty, Kitty, iTerm2, WezTerm, Windows Terminal)