Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/styles.md

@@ -0,0 +1,21 @@
+# Headers, primary, and secondary text
+
+- **Headers:** Use `bold`. For markdown with various header levels, leave in the `#` signs.
+- **Primary text:** Default.
+- **Secondary text:** Use `dim`.
+
+# Foreground colors
+
+- **Default:** Most of the time, just use the default foreground color. `reset` can help get it back.
+- **User input tips, selection, and status indicators:** Use ANSI `cyan`.
+- **Success and additions:** Use ANSI `green`.
+- **Errors, failures and deletions:** Use ANSI `red`.
+- **Codex:** Use ANSI `magenta`.
+
+# Avoid
+
+- Avoid custom colors because there's no guarantee that they'll contrast well or look good in various terminal color themes. (`shimmer.rs` is an exception that works well because we take the default colors and just adjust their levels.)
+- Avoid ANSI `black` & `white` as foreground colors because the default terminal theme color will do a better job. (Use `reset` if you need to in order to get those.) The exception is if you need contrast rendering over a manually colored background.
+- Avoid ANSI `blue` and `yellow` because for now the style guide doesn't use them. Prefer a foreground color mentioned above.
+
+(There are some rules to try to catch this in `clippy.toml`.)