Expose Codex skills as ACP slash commands

[benjaminshoemaker]

Summary

Refs zed-industries/zed#55983.

This teaches codex-acp to publish enabled Codex skills as ACP available commands using Codex's native $skill-name invocation form.

Because Zed's Agent Panel discovers commands through ACP slash commands, the adapter exposes skills as /$skill-name completions and rewrites submitted /$skill-name ... prompts to $skill-name ... before forwarding them to Codex core.

Testing

• cargo fmt -- --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• Manually tested in Zed Agent Panel with a locally swapped codex-acp binary:
  • Opened an external Codex CLI thread
  • Verified the composer placeholder says / for commands
  • Typed / and saw command completions
  • Typed /$ and saw installed Codex skills such as $go, $ui-ux, and $progress
  • Did not submit a real prompt

Notes

This intentionally reuses Codex's existing skill loading path and invocation syntax rather than inventing separate ACP-only skill discovery.

Release Notes:

• Improved Codex skill discoverability in ACP clients that expose slash command completions.

[benjaminshoemaker]

Here's what it looks like in agent mode