* feat(cli): /prompt — compose your next prompt in $EDITOR
Adds /prompt (alias /compose): opens $VISUAL/$EDITOR on a temp markdown
file so you can hand-edit a multi-line prompt, then sends the saved buffer
as the next agent turn. Text after the command pre-seeds the buffer; an
empty save cancels. Reuses the one-shot _pending_agent_seed the interactive
loop already consumes (same mechanism as /blueprint), so no changes to the
input event loop or message pipeline. CLI-only.
* feat(tui): /prompt slash command opens $EDITOR (parity with CLI)
The TUI already opens $EDITOR via Ctrl+G (openEditor), but had no /prompt
slash command like the classic CLI. Wire openEditor into the slash handler
context and register /prompt (alias /compose) to call it; inline text after
the command is dropped into the composer first so it carries into the editor,
matching the CLI's /prompt <text>.
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