test: quarantine the watch SIGINT subprocess test to the serial lane

The parallel unit shards flake on concurrent CLI subprocess spawns (failed
at 7ms in-suite, green solo) — same isolation rationale as
apply-migrations-pglite-spawn.serial.test.ts and #2141's R3 quarantine.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/watch-command.test.ts

@@ -153,48 +153,3 @@ describe('gbrain watch — window + cap flags (ship coverage G4)', () => {
     expect(out.length).toBe(1);
   });
 });
-
-describe('gbrain watch — SIGINT lifecycle (real subprocess)', () => {
-  test('SIGINT mid-stream closes the stream and exits cleanly (drain path, exit 0)', async () => {
-    const { mkdtempSync, mkdirSync, writeFileSync, rmSync } = await import('fs');
-    const { join, resolve } = await import('path');
-    const { tmpdir } = await import('os');
-    const REPO = resolve(import.meta.dir, '..');
-    const home = mkdtempSync(join(tmpdir(), 'gbrain-watch-sigint-'));
-    try {
-      mkdirSync(join(home, '.gbrain'), { recursive: true });
-      writeFileSync(
-        join(home, '.gbrain', 'config.json'),
-        JSON.stringify({
-          engine: 'pglite',
-          database_path: join(home, '.gbrain', 'brain.pglite'),
-          embedding_dimensions: 1536,
-        }) + '\n',
-      );
-      // Piped stdin that NEVER reaches EOF — only SIGINT can end the stream.
-      const proc = Bun.spawn(['bun', 'run', join(REPO, 'src', 'cli.ts'), 'watch'], {
-        cwd: REPO,
-        env: { ...process.env, HOME: home, GBRAIN_HOME: home, GBRAIN_SKIP_STARTUP_HOOKS: '1' },
-        stdin: 'pipe',
-        stdout: 'pipe',
-        stderr: 'pipe',
-      });
-      proc.stdin.write('user: nothing relevant here\n');
-      await proc.stdin.flush();
-      // Give the brain time to init + process the turn, then interrupt.
-      await new Promise((r) => setTimeout(r, 15_000));
-      proc.kill('SIGINT');
-      const killer = setTimeout(() => {
-        try { proc.kill('SIGKILL'); } catch { /* already dead */ }
-      }, 30_000);
-      const exitCode = await proc.exited;
-      clearTimeout(killer);
-      const stderr = await new Response(proc.stderr).text();
-      // Clean drain-then-exit: no force-exit banner, no SIGKILL (137), exit 0.
-      expect(stderr).not.toContain('force-exiting');
-      expect(exitCode).toBe(0);
-    } finally {
-      rmSync(home, { recursive: true, force: true });
-    }
-  }, 120_000);
-});

test/watch-sigint.serial.test.ts

@@ -0,0 +1,52 @@
+/**
+ * v0.43 (#2095) — `gbrain watch` SIGINT lifecycle. SERIAL: spawns a real CLI
+ * subprocess with a tmpdir brain (the parallel unit shards flake on
+ * concurrent subprocess spawns — same isolation rationale as
+ * apply-migrations-pglite-spawn.serial.test.ts).
+ */
+import { describe, test, expect } from 'bun:test';
+
+describe('gbrain watch — SIGINT lifecycle (real subprocess)', () => {
+  test('SIGINT mid-stream closes the stream and exits cleanly (drain path, exit 0)', async () => {
+    const { mkdtempSync, mkdirSync, writeFileSync, rmSync } = await import('fs');
+    const { join, resolve } = await import('path');
+    const { tmpdir } = await import('os');
+    const REPO = resolve(import.meta.dir, '..');
+    const home = mkdtempSync(join(tmpdir(), 'gbrain-watch-sigint-'));
+    try {
+      mkdirSync(join(home, '.gbrain'), { recursive: true });
+      writeFileSync(
+        join(home, '.gbrain', 'config.json'),
+        JSON.stringify({
+          engine: 'pglite',
+          database_path: join(home, '.gbrain', 'brain.pglite'),
+          embedding_dimensions: 1536,
+        }) + '\n',
+      );
+      // Piped stdin that NEVER reaches EOF — only SIGINT can end the stream.
+      const proc = Bun.spawn(['bun', 'run', join(REPO, 'src', 'cli.ts'), 'watch'], {
+        cwd: REPO,
+        env: { ...process.env, HOME: home, GBRAIN_HOME: home, GBRAIN_SKIP_STARTUP_HOOKS: '1' },
+        stdin: 'pipe',
+        stdout: 'pipe',
+        stderr: 'pipe',
+      });
+      proc.stdin.write('user: nothing relevant here\n');
+      await proc.stdin.flush();
+      // Give the brain time to init + process the turn, then interrupt.
+      await new Promise((r) => setTimeout(r, 15_000));
+      proc.kill('SIGINT');
+      const killer = setTimeout(() => {
+        try { proc.kill('SIGKILL'); } catch { /* already dead */ }
+      }, 30_000);
+      const exitCode = await proc.exited;
+      clearTimeout(killer);
+      const stderr = await new Response(proc.stderr).text();
+      // Clean drain-then-exit: no force-exit banner, no SIGKILL (137), exit 0.
+      expect(stderr).not.toContain('force-exiting');
+      expect(exitCode).toBe(0);
+    } finally {
+      rmSync(home, { recursive: true, force: true });
+    }
+  }, 120_000);
+});