tui: warn on Ultra with high multi-agent concurrency

[shijie-oai]

Why

Ultra reasoning may proactively use multiple agents. When features.multi_agent_v2.max_concurrent_threads_per_session is configured at 8 or higher, explicitly selecting Ultra can allow up to N - 1 subagents to work concurrently and increase usage quickly. Showing the configured limits at selection time makes that tradeoff visible and points users to the setting that controls it.

What changed

• Show a warning history cell after the user explicitly selects Ultra reasoning with a concurrent-thread limit of at least 8.
• Include the configured concurrent-thread count and maximum subagent count in the warning.
• Apply the warning consistently across model and reasoning pickers, Plan-mode scope selection, and reasoning shortcuts.
• Keep the trigger limited to the selected reasoning effort and configured thread limit, independent of how multi-agent v2 is activated.
• Add focused threshold coverage and an insta snapshot for the rendered warning.

User impact

Users with high multi-agent concurrency receive a concrete warning immediately after selecting Ultra. Other reasoning efforts, limits below 8, and startup behavior are unchanged.

Screen.Recording.2026-07-08.at.11.07.56.AM.mov

Test plan

• just test -p codex-tui ultra_reasoning_selection
• Built the full debug CLI with cargo build -p codex-cli.
• Manually verified the debug TUI with a 10-thread limit: switching from Max to Ultra displayed the warning with 10 concurrent threads and up to 9 subagents.

[fcoury-oai]

Smoke tested and worked as expected. Code looks good, approved 👍