Restore slash command support on rust-v0.42.0

[iatzmon]

Summary

• add the codex-slash-commands crate to the workspace and expose a service in codex-core for resolving custom slash commands
• enable slash command support in the core submission loop, including reload handling and command interpolation, and wire the feature through the CLI/TUI crates
• extend the protocol with a ReloadSlashCommands op so clients can refresh the registry

Testing

• cargo fmt
• cargo check -p codex-slash-commands

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https://chatgpt.com/codex/tasks/task_e_68d93868cbc0832a9fa425b6e3b90df5

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch codex/rebase-branch-on-codex-and-add-slash-commands

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