feat(thread): add /fast command to toggle Fast mode

[Shvandre]

Closes #274

[rohitpaulk]

Has anyone found a workaround for this? Is there a way to boot the Codex ACP server with an environment variable or so?

[rohitpaulk]

Answered my own question: you can set this in ~/.codex/config.toml and fast mode will be picked by default:

[features]
fast_mode = true

[phplego]

I think this is the right scoped split for fast mode support.

This PR is small and focused:

• one command: /fast
• two files touched: README.md, src/thread.rs
• maps to Codex ServiceTier::Fast via OverrideTurnContext
• keeps /fast on, /fast off, and /fast status covered by unit tests

I checked the branch locally:

• cargo fmt -- --check: passed
• CARGO_HOME=/tmp/codex-acp-cargo cargo test test_fast -- --nocapture: passed
• result: 3 passed, 0 failed

The GitHub Actions run currently appears to be action_required rather than failed, so it likely just needs maintainer approval to run CI.

This also lines up with the maintainer feedback on #206 asking for smaller scoped feature PRs.

[osimarr]

Oh I see my PR is possibly a duplicate.
I took a different approach: instead of /fast command, it shows GPT-5.5/4 and GPT-5.5/4-Fast as different models