feat: make it possible to toggle mouse mode in the Rust TUI (#971)

I did a bit of research to understand why I could not use my mouse to
drag to select text to copy to the clipboard in iTerm.

Apparently https://github.com/openai/codex/pull/641 to enable mousewheel
scrolling broke this functionality. It seems that, unless we put in a
bit of effort, we can have drag-to-select or scrolling, but not both.
Though if you know the trick to hold down `Option` will dragging with
the mouse in iTerm, you can probably get by with this. (I did not know
about this option prior to researching this issue.)

Nevertheless, users may still prefer to disable mouse capture
altogether, so this PR introduces:

* the ability to set `tui.disable_mouse_capture = true` in `config.toml`
to disable mouse capture
* a new command, `/toggle-mouse-mode` to toggle mouse capture

codex-rs/README.md

@@ -329,3 +329,21 @@ Currently, `"vscode"` is the default, though Codex does not verify VS Code is in
 ### project_doc_max_bytes
 
 Maximum number of bytes to read from an `AGENTS.md` file to include in the instructions sent with the first turn of a session. Defaults to 32 KiB.
+
+### tui
+
+Options that are specific to the TUI.
+
+```toml
+[tui]
+# This will make it so that Codex does not try to process mouse events, which
+# means your Terminal's native drag-to-text to text selection and copy/paste
+# should work. The tradeoff is that Codex will not receive any mouse events, so
+# it will not be possible to use the mouse to scroll conversation history.
+#
+# Note that most terminals support holding down a modifier key when using the
+# mouse to support text selection. For example, even if Codex mouse capture is
+# enabled (i.e., this is set to `false`), you can still hold down alt while
+# dragging the mouse to select text.
+disable_mouse_capture = true  # defaults to `false`
+```