[codex] Alphabetize CLI help commands

[sm18lr88]

What changed

• Alphabetizes the top-level rtk --help command menu by assigning equal Clap display order to every user-facing top-level subcommand.
• Adds a regression test that renders the long help and verifies non-built-in top-level commands are sorted alphabetically.

Why

The CLI help menu was emitted in enum definition order, which made the command list hard to scan as more wrappers were added. Clap sorts commands that share the same display order, so setting a shared display order keeps runtime dispatch unchanged while producing an organized menu.

Validation

• cargo fmt --check
• rtk cargo check
• rtk cargo test test_top_level_help_commands_are_alphabetical
• cargo run --quiet -- --help

Full rtk cargo test was also run and currently fails on Windows in core::stream tests because test commands such as Unix-style echo/cat/exit helpers are not found on PATH; unrelated tests passed (2050 passed; 16 failed; 7 ignored).

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