Add /effort slash command for reasoning effort selection

Summary

I would like Codex to provide a direct /effort slash command for switching the reasoning effort of the current model.

Today /model can change both the model and reasoning effort, but it always starts with model selection first. For users who only want to adjust effort, a dedicated /effort command would be more direct.

Proposed behavior

• /effort opens the existing reasoning-level picker for the current model.
• It reuses the same four levels already shown by /model: low, medium, high, and extra high.
• It should be unavailable while a task is running, matching /model behavior.
• Inline arguments such as /effort high can be considered later, but are not required for the minimal version.

Prototype branch

I implemented a minimal prototype here:

• Branch: https://github.com/qiankunli/codex/tree/feature/effort-command
• Compare: https://github.com/openai/codex/compare/main...qiankunli:codex:feature/effort-command?expand=1

The prototype adds the slash command, reuses the existing reasoning picker UI, and includes a snapshot test.

Testing done on the prototype

• cargo fmt
• cargo test -p codex-tui effort_slash_command_opens_current_model_reasoning_popup_snapshot
• cargo test -p codex-tui

Note

I could not create a pull request from the fork branch to openai/codex:main with my current GitHub permissions, so I am filing this issue with the branch link instead.