[codex] Fix Vim normal mode editing

[jinghanx88]

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

[github-actions]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid deleting line breaks for s on empty lines

When the cursor is on an empty Vim line, beginning_of_current_line() and end_of_current_line() are equal and the cursor points at the newline byte, so this new s path calls delete_forward_kill(1) and removes that newline before entering insert mode. In practice, pressing s on a blank line collapses it into the next line instead of just entering insert mode on the empty line, which is destructive for normal Vim editing of multi-line prompts.

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[jinghanx88]

I have read the CLA Document and I hereby sign the CLA

[jinghanx88]

recheck