fix(auto-compact): send a multi-line focus prompt as one line

Auto-compact writes `/compact <prompt>\r` through writeViaMux when the token
count crosses the threshold, unattended. AutoCompactSchema.prompt accepts line
breaks, so a prompt set through the API can hold several lines. The mux used to
strip them and run the lines together; it now pastes multi-line text as one
block, and the check that re-presses a stranded Enter cannot recognise that
paste in the composer. That is the case respawn's update and kickstart prompts
were already joined for.

The join moves out of respawn-controller into a shared toSingleLine()
(src/utils/single-line.ts), used by both respawn steps and by auto-compact.
Behaviour for respawn is unchanged. The respawn test file becomes
test/unattended-single-line-prompt.test.ts and gains an auto-compact block that
feeds a real Session a token count over its threshold and reads what reaches
writeViaMux.

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`) 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.
+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's update and kickstart prompts and the auto-compact focus prompt now have their lines joined with a space (they used to run together), and `codeman agent send` and the TUI keep refusing or joining multi-line input.