Add Codex activity module

[oswald-wang-tw]

Summary

• Add a new codex module that simulates a Codex-style coding session
• Register the module in ALL_MODULES and document it in the README module list
• Include colorful terminal transcript output with streamed thinking notes, planning, tool activity, diffs, and verification

Testing

• cargo fmt --all
• cargo check
• cargo clippy -- -D warnings
• cargo test
• cargo run -- --modules codex --speed-factor 120 --exit-after-modules 1
• cargo run -- --list-modules

[svenstaro]

If you used an LLM to create this, disclose the model, prompt and harness here, please.

[svenstaro]

Did you also check the web view for the wasm target?

[svenstaro]

Also, a screenshot, apng or gif would be nice to attach to the PR.