feat(cli): add /editor to compose prompts in $EDITOR

[emal-avala]

Summary

New /editor slash command (alias /ed) that opens the user's editor on a tempfile, then submits the contents as the user turn when the editor exits.

> /editor
(vim opens; write multi-paragraph prompt; :wq)
<agent receives the composed prompt and responds>

> /editor initial draft text
(editor opens pre-filled with "initial draft text")

Why

rustyline is great for one-liners but painful for multi-paragraph prompts — \ continuations and no way to scroll back through what you've typed. `/editor` solves the most-requested readline limitation: compose long prompts in your real editor where tab/indent/copy-paste all work.

Editor resolution

1. $VISUAL
2. $EDITOR
3. vim if on PATH
4. vi if on PATH
5. nano if on PATH

Returns a helpful error if none are found.

Behaviour details

• Empty content → "Editor closed with empty content; nothing to send." (no turn submitted)
• Lines starting with # are stripped (so the pre-fill hint at the bottom of a blank file doesn't leak into the prompt)
• Pre-fill supported: /editor first draft text opens the editor already containing that text
• Tempfile uses .md suffix so editors with markdown modes get syntax highlighting
• Non-zero exit from the editor aborts (no prompt submitted)

Test plan

• `cargo fmt --all` — clean
• `cargo clippy --workspace --all-targets -- -D warnings` — clean
• `cargo test -p agent-code --bin agent -- resolve_editor which_in_path` — 5/5 pass
  • `resolve_editor_prefers_visual` (VISUAL beats EDITOR)
  • `resolve_editor_falls_back_to_editor_env` (uses EDITOR when VISUAL unset)
  • `resolve_editor_ignores_empty_env` (empty/whitespace strings don't leak through)
  • `which_in_path_finds_shell` (sanity check: /bin/sh exists on *nix, cmd on Windows)
  • `which_in_path_rejects_missing` (nonexistent binary)
• Manual: with `EDITOR=vim`, `/editor` opens an empty buffer with a hint; writing content and `:wq` submits; `:q!` (no save) cancels; `/editor pre` pre-fills with `pre`

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.