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>

src/utils/index.ts

@@ -27,6 +27,7 @@ export { MAX_SESSION_TOKENS } from './token-validation.js';
 export { isSafePushEndpoint } from './push-endpoint-validation.js';
 export { stringSimilarity, fuzzyPhraseMatch, todoContentHash } from './string-similarity.js';
 export { assertNever } from './type-safety.js';
+export { toSingleLine } from './single-line.js';
 export { wrapWithNice } from './nice-wrapper.js';
 export { resolveLocalShell, loginShellArgs } from './shell-resolver.js';
 export {

src/utils/single-line.ts

@@ -0,0 +1,20 @@
+/**
+ * @fileoverview Collapse a prompt onto the one line an unattended sender writes.
+ *
+ * @module utils/single-line
+ */
+
+/**
+ * A prompt as one line: every run of line breaks, with the whitespace around it,
+ * becomes a single space; a single-line prompt is returned as is.
+ *
+ * For the senders that run unattended through `writeViaMux` (respawn's update and
+ * kickstart steps, auto-compact). The mux pastes a multi-line prompt as one block,
+ * and the check that re-presses a stranded Enter cannot recognise a multi-line paste
+ * in the composer, so nobody would notice a prompt left unsubmitted. Cron rejects
+ * multi-line prompts for the same reason; these senders join them instead, so a
+ * saved multi-line setting is still sent rather than refused.
+ */
+export function toSingleLine(prompt: string): string {
+  return prompt.replace(/\s*[\r\n]+\s*/g, ' ');
+}