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>

test/unattended-single-line-prompt.test.ts

@@ -5,9 +5,9 @@
  * auto-compact focus prompt's schema accepts line breaks, so a saved config can hold
  * several lines. `TmuxManager.sendInput()` used to strip every `\n`, which ran the
  * lines together; it now pastes multi-line text as one block. These senders run
- * unattended, and the check that re-presses a stranded Enter cannot recognise a
- * multi-line paste in the composer, so they join the lines with a space before the
- * write (`toSingleLine()`) instead of leaning on the mux to drop them.
+ * unattended, and the check that re-presses a stranded Enter cannot reliably
+ * recognise a multi-line paste in the composer, so they join the lines with a space
+ * before the write (`toSingleLine()`) instead of leaning on the mux to drop them.
  *
  * Strategy: drive the respawn step methods against a MockSession, and feed a real
  * Session a token count over its auto-compact threshold, then read what reached
@@ -37,6 +37,22 @@ describe('toSingleLine', () => {
   it('returns a single-line prompt unchanged', () => {
     expect(toSingleLine('  keep "quotes" and $vars  ')).toBe('  keep "quotes" and $vars  ');
   });
+
+  it('drops leading and trailing blank lines instead of leaving a space there', () => {
+    expect(toSingleLine('\n  compact the log  \n\n')).toBe('compact the log');
+  });
+
+  it('stays linear on a long whitespace run', () => {
+    // A regex that folds the whitespace around each break backtracks over every
+    // start position in the run: 50,000 spaces took well over a second that way.
+    // The split takes well under a millisecond, so the bound leaves room either side.
+    const run = ' '.repeat(50_000);
+    const started = performance.now();
+    const joined = toSingleLine(`a${run}b\nc${run}`);
+    const elapsed = performance.now() - started;
+    expect(joined).toBe(`a${run}b c`);
+    expect(elapsed).toBeLessThan(250);
+  });
 });
 
 describe('respawn steps send a multi-line prompt as one line', () => {