Feature request: support custom slash commands in interactive CLI

Summary

Please add support for user-defined slash commands in Codex interactive mode (the / command palette).

Problem

Today, slash commands appear to be built-in only.
Users who run repetitive workflows cannot register project-specific commands (e.g. /fix-tests, /review-pr, /release-notes) and must repeatedly type long prompts/options.

Proposed solution

Add a configurable custom slash command system.

Suggested UX

• Define commands in config (global and project-level), e.g. ~/.codex/config.toml and optional repo-local file.
• Commands appear in / autocomplete menu with descriptions.
• Support parameter placeholders (e.g. $1, ${branch}).
• Command body can:
• inject prompt text
• optionally attach fixed flags/profile
• optionally run a predefined local script (with existing approval/sandbox model)

Example (illustrative)

[slash_commands.fix_tests]
description = "Analyze and fix failing tests"
prompt = "Run tests, identify root causes, apply minimal fixes, and rerun."

[slash_commands.review_pr]
description = "Review current branch diff"
prompt = "Perform a code review focused on regressions, security, and missing tests."

Why this matters

• Faster repeated workflows
• Better team standardization
• Lower prompt friction
• Closer parity with tools that already support custom slash commands

Backward compatibility

No breaking changes expected. If no custom commands are configured, behavior remains unchanged.