Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/tui/src/exec_command.rs

@@ -1,70 +0,0 @@
-use std::path::Path;
-use std::path::PathBuf;
-
-use codex_shell_command::parse_command::extract_shell_command;
-use dirs::home_dir;
-use shlex::try_join;
-
-pub(crate) fn escape_command(command: &[String]) -> String {
-    try_join(command.iter().map(String::as_str)).unwrap_or_else(|_| command.join(" "))
-}
-
-pub(crate) fn strip_bash_lc_and_escape(command: &[String]) -> String {
-    if let Some((_, script)) = extract_shell_command(command) {
-        return script.to_string();
-    }
-    escape_command(command)
-}
-
-/// If `path` is absolute and inside $HOME, return the part *after* the home
-/// directory; otherwise, return the path as-is. Note if `path` is the homedir,
-/// this will return and empty path.
-pub(crate) fn relativize_to_home<P>(path: P) -> Option<PathBuf>
-where
-    P: AsRef<Path>,
-{
-    let path = path.as_ref();
-    if !path.is_absolute() {
-        // If the path is not absolute, we can’t do anything with it.
-        return None;
-    }
-
-    let home_dir = home_dir()?;
-    let rel = path.strip_prefix(&home_dir).ok()?;
-    Some(rel.to_path_buf())
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn test_escape_command() {
-        let args = vec!["foo".into(), "bar baz".into(), "weird&stuff".into()];
-        let cmdline = escape_command(&args);
-        assert_eq!(cmdline, "foo 'bar baz' 'weird&stuff'");
-    }
-
-    #[test]
-    fn test_strip_bash_lc_and_escape() {
-        // Test bash
-        let args = vec!["bash".into(), "-lc".into(), "echo hello".into()];
-        let cmdline = strip_bash_lc_and_escape(&args);
-        assert_eq!(cmdline, "echo hello");
-
-        // Test zsh
-        let args = vec!["zsh".into(), "-lc".into(), "echo hello".into()];
-        let cmdline = strip_bash_lc_and_escape(&args);
-        assert_eq!(cmdline, "echo hello");
-
-        // Test absolute path to zsh
-        let args = vec!["/usr/bin/zsh".into(), "-lc".into(), "echo hello".into()];
-        let cmdline = strip_bash_lc_and_escape(&args);
-        assert_eq!(cmdline, "echo hello");
-
-        // Test absolute path to bash
-        let args = vec!["/bin/bash".into(), "-lc".into(), "echo hello".into()];
-        let cmdline = strip_bash_lc_and_escape(&args);
-        assert_eq!(cmdline, "echo hello");
-    }
-}