[codex] Fix Vim normal mode editing (#25022)

## Summary
- add Vim normal-mode `s` support to substitute the character under the
cursor and enter insert mode
- fix Vim normal-mode `o` so opening below the final line moves the
cursor onto the new blank line
- update keymap config/schema and keymap picker snapshots for the new
action

## Validation
- `just fmt`
- `just write-config-schema`
- `just test -p codex-config`
- focused `just test -p codex-tui` coverage for the Vim `s` and `o`
behavior, keymap conflict handling, and keymap picker snapshots
- `cargo insta pending-snapshots --manifest-path tui/Cargo.toml`
- `git diff --check`

## Notes
A full `just test -p codex-tui` run still has two unrelated Guardian
feature-flag failures in this checkout:
-
`app::tests::update_feature_flags_disabling_guardian_clears_review_policy_and_restores_default`
-
`app::tests::update_feature_flags_disabling_guardian_clears_manual_review_policy_without_history`