fix(respawn): send a multi-line update or kickstart prompt as one line

The update and kickstart prompts are textareas in the respawn settings and
the schema accepts line breaks, so a saved config can hold several lines.
Both steps went out through writeViaMux, which used to strip every `\n` and
run the lines together. Since the mux now pastes multi-line text as one
block, a multi-line respawn prompt would have gone out as a multi-line paste,
and the check that re-presses a stranded Enter cannot recognise that paste in
the composer. Respawn runs unattended, the same situation for which cron
keeps rejecting multi-line prompts.

singleLineRespawnPrompt() joins the lines with a space before the write; a
single-line prompt is passed through unchanged. Respawn joins rather than
rejects, so a multi-line config that was accepted before is still sent.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.changeset/multiline-input-paste.md

@@ -2,4 +2,4 @@
 'aicodeman': patch
 ---
 
-A multi-line prompt sent through the mux (`POST /api/sessions/:id/input` with `useMux`, respawn prompts) now keeps its line breaks. `sendInput` used to strip every `\n` before the text reached tmux, so `"line one\nline two\r"` arrived as `line oneline two`. The lines now go in as one bracketed paste, the same paste a single-line prompt already uses, and the Enter after it submits them together; multi-line text without an Enter is pasted too instead of typed. A shell runs each line. The submit check that re-presses a stranded Enter does not yet recognise a multi-line paste in the composer, so `codeman agent send` and the TUI keep refusing or joining multi-line input.
+A multi-line prompt sent through the mux (`POST /api/sessions/:id/input` with `useMux`) now keeps its line breaks. `sendInput` used to strip every `\n` before the text reached tmux, so `"line one\nline two\r"` arrived as `line oneline two`. The lines now go in as one bracketed paste, the same paste a single-line prompt already uses, and the Enter after it submits them together; multi-line text without an Enter is pasted too instead of typed. A shell runs each line. The submit check that re-presses a stranded Enter does not yet recognise a multi-line paste in the composer, so the unattended senders stay single-line: cron still rejects multi-line prompts, respawn now joins the lines of a multi-line update or kickstart prompt with a space (they used to run together), and `codeman agent send` and the TUI keep refusing or joining multi-line input.