Trim TUI legacy core helper usage

[etraut-openai]

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