config: accept minus in TUI keymap config

[fcoury-oai]

Summary

Fixes #22128.

The /keymap flow already persists the - key as minus, and the runtime keymap parser already accepts that spelling. codex-config was the missing leg: it rejected minus during config deserialization, so a binding saved by Codex could fail on the next startup or config reload.

What Changed

• Accept minus as a valid canonical key name in tui.keymap config normalization.
• Update the config validation message so its supported-key list includes minus.
• Add regression coverage that deserializes both minus and alt-minus under [tui.keymap.global] and verifies the normalized config shape.

How to Test

1. Start Codex TUI.
2. Run /keymap.
3. Assign the - key to an action and save the change.
4. Restart Codex or reload the config.
5. Confirm the config loads normally and the saved binding remains usable instead of failing on minus.
6. As a focused regression check, repeat with a modifier form such as alt-- captured through /keymap, which persists as alt-minus and should also reload successfully.

Targeted tests:

• cargo test -p codex-config