BUG: Custom prompts are advertised but not available through codex-acp #289

Version

• @zed-industries/[email protected]
• Release: v0.14.0

Problem

• README advertises Custom Prompts under slash commands.
• ACP command list only publishes hard-coded built-ins.
• Prompt handling only handles those built-ins.
• Unknown slash commands fall through as plain user input.

Repro

1. Create ~/.codex/prompts/foo.md.
2. Start Codex through codex-acp.
3. Open ACP client slash command picker.
4. Observe /foo is absent.
5. Send /foo.

Expected

• Custom prompts from Codex prompt dirs are listed.
• /foo [args] expands prompt content before submission.

Actual

• Only review, review-branch, review-commit, init, compact, logout are listed.
• Unknown slash commands are submitted as Op::UserInput.

Source refs

• README advertises custom prompts:
  https://github.com/zed-industries/codex-acp/blob/v0.14.0/README.md#L13-L20
• Command list sends only Self::builtin_commands():
  https://github.com/zed-industries/codex-acp/blob/v0.14.0/src/thread.rs#L2822-L2824
• builtin_commands() is hard-coded:
  https://github.com/zed-industries/codex-acp/blob/v0.14.0/src/thread.rs#L2894-L2924
• handle_prompt() falls unknown slash commands back to Op::UserInput:
  https://github.com/zed-industries/codex-acp/blob/v0.14.0/src/thread.rs#L3244-L3317