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>

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

@@ -0,0 +1,93 @@
+/**
+ * Respawn prompts go out as ONE line, whatever the config holds.
+ *
+ * The update and kickstart prompts are textareas in the settings, 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. Respawn runs
+ * unattended, and the check that re-presses a stranded Enter cannot recognise a
+ * multi-line paste in the composer, so the controller joins the lines with a space
+ * before the write instead of leaning on the mux to drop them.
+ *
+ * Strategy: drive the step methods directly against a MockSession and read what
+ * reached `writeViaMux`. No real tmux.
+ *
+ * Port: N/A (no server / no real tmux)
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+vi.mock('node:child_process', async (orig) => {
+  const actual = await orig<typeof import('node:child_process')>();
+  return { ...actual, exec: vi.fn((_cmd: string, cb?: (e: Error | null, o: string) => void) => cb?.(null, '')) };
+});
+
+import { RespawnController, singleLineRespawnPrompt } from '../src/respawn-controller.js';
+import { Session } from '../src/session.js';
+import { MockSession } from './mocks/index.js';
+
+describe('singleLineRespawnPrompt', () => {
+  it('joins lines with one space, folding the whitespace around each break', () => {
+    expect(singleLineRespawnPrompt('update the docs\n\n  then CLAUDE.md\r\nand continue')).toBe(
+      'update the docs then CLAUDE.md and continue'
+    );
+  });
+
+  it('returns a single-line prompt unchanged', () => {
+    expect(singleLineRespawnPrompt('  keep "quotes" and $vars  ')).toBe('  keep "quotes" and $vars  ');
+  });
+});
+
+describe('respawn steps send a multi-line prompt as one line', () => {
+  let controller: RespawnController | null = null;
+
+  afterEach(() => controller?.stop());
+
+  /** Run one step method and return what it wrote through the mux. */
+  async function written(
+    config: { updatePrompt?: string; kickstartPrompt?: string },
+    step: 'sendUpdateDocs' | 'sendKickstart',
+    recommendation?: string
+  ): Promise<string[]> {
+    const session = new MockSession();
+    if (recommendation !== undefined) {
+      (session as unknown as { ralphTracker: unknown }).ralphTracker = { lastStatusBlock: { recommendation } };
+    }
+    controller = new RespawnController(session as unknown as Session, {
+      interStepDelayMs: 10,
+      completionConfirmMs: 10,
+      noOutputTimeoutMs: 500,
+      aiIdleCheckEnabled: false,
+      ...config,
+    });
+    (controller as never as { _state: string })._state = 'watching';
+    (controller as never as Record<typeof step, () => void>)[step]();
+    // The step schedules its write on the step-delay timer.
+    await new Promise((r) => setTimeout(r, 40));
+    return session.writeBuffer;
+  }
+
+  it('update prompt', async () => {
+    const writes = await written(
+      { updatePrompt: 'summarise progress\nin CLAUDE.md\n\nthen continue' },
+      'sendUpdateDocs'
+    );
+    expect(writes).toEqual(['summarise progress in CLAUDE.md then continue\r']);
+  });
+
+  it('update prompt with a recommendation appended', async () => {
+    const writes = await written({ updatePrompt: 'summarise\nprogress' }, 'sendUpdateDocs', 'fix the\nlogin test');
+    expect(writes).toEqual(["summarise progress (Claude's last recommendation: fix the login test)\r"]);
+  });
+
+  it('kickstart prompt', async () => {
+    const writes = await written(
+      { kickstartPrompt: 'read @fix_plan.md\r\ncontinue with the next task' },
+      'sendKickstart'
+    );
+    expect(writes).toEqual(['read @fix_plan.md continue with the next task\r']);
+  });
+
+  it('a single-line prompt is sent as before', async () => {
+    const writes = await written({ updatePrompt: 'update all the docs and CLAUDE.md' }, 'sendUpdateDocs');
+    expect(writes).toEqual(['update all the docs and CLAUDE.md\r']);
+  });
+});