Add /edit external-editor workflow with command parsing, busy UI, and docs/tests updates

[honjo-hiroaki-gtt]

• this follows collaborator feedback in PR #3044, comment 3309270038, which suggested adding a keyboard shortcut to open the current prompt in $EDITOR
• introduce /edit slash command and Ctrl+E shortcut that resolve $VISUAL → $EDITOR → OS fallback and safely round-trip the composer through an external editor (--send, --new, --keep size limit, kill switch)
• integrate the workflow with the TUI: preserve attachments when reloading, surface inline busy state while waiting on the editor, and gate the feature behind CODEX_EDIT_DISABLE
• factor command parsing via SlashCommandInvocation, update ChatWidget dispatch, and keep composer placeholders in sync
• update README/config docs and extend unit tests (slash command parsing, external editor mock paths); cargo test -p codex-tui passes locally

[chatgpt-codex-connector]

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