fix(single-line): join lines by split and trim instead of a backtracking regex

/\s*[\r\n]+\s*/g backtracks over every start position of a long
whitespace run, so 50,000 spaces took over a second. Split on line breaks,
trim each line, drop blank ones and join with a space: linear, and a
leading or trailing blank line no longer leaves a space behind. A prompt
without a line break is still returned unchanged. session-auto-ops now
imports toSingleLine from the utils barrel like respawn-controller.

Co-Authored-By: Christian Haberl <116439789+christianhaberl@users.noreply.github.com>
Co-Authored-By: Claude <noreply@anthropic.com>

src/utils/single-line.ts

@@ -5,16 +5,25 @@
  */
 
 /**
- * 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.
+ * A prompt as one line: each line is trimmed, blank lines are dropped, and the rest
+ * are joined with a single space; a prompt without a line break 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.
+ * and the check that re-presses a stranded Enter cannot reliably 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.
+ *
+ * Split and trim, which is linear, rather than one regex replace that folds the
+ * whitespace around each break: that regex backtracks quadratically over a long
+ * whitespace run (50,000 spaces took over a second).
  */
 export function toSingleLine(prompt: string): string {
-  return prompt.replace(/\s*[\r\n]+\s*/g, ' ');
+  if (!/[\r\n]/.test(prompt)) return prompt;
+  return prompt
+    .split(/[\r\n]+/)
+    .map((line) => line.trim())
+    .filter(Boolean)
+    .join(' ');
 }