Trim TUI legacy core helper usage (#22695)

## Why

The TUI still had a few low-risk dependencies flowing through the
transitional `legacy_core` namespace after the app-server migration.
These helpers either already have clearer non-core owners or are
presentation logic that does not belong in `codex-core`, so moving them
out reduces the compatibility surface without changing product behavior.

## What changed

This is a low-risk change, almost completely mechanical in nature.

- Route TUI Codex-home lookup through `codex-utils-home-dir`, use
`Config::log_dir` directly, and call
`codex-sandboxing::system_bwrap_warning` without going through
`legacy_core`.
- Move shared `codex resume` hint formatting from `codex-core` into
`codex-utils-cli`.
- Update CLI and TUI call sites to use the shared CLI utility, and keep
the resume-command behavior covered by tests in its new home.

## Verification

- `cargo test -p codex-utils-cli`
- `cargo test -p codex-utils-cli resume_command`

codex-rs/tui/src/chatwidget/settings_popups.rs

@@ -7,7 +7,7 @@ use super::*;
 
 impl ChatWidget {
     pub(super) fn open_theme_picker(&mut self) {
-        let codex_home = crate::legacy_core::config::find_codex_home().ok();
+        let codex_home = codex_utils_home_dir::find_codex_home().ok();
         let terminal_width = self
             .last_rendered_width
             .get()