fix(3597): make windows test cleanup retry on EBUSY + drop CRLF-broken local parseFrontmatter

Two windows-only failure clusters surfaced when the chunking fix in
52f23ac0 made Windows tests actually run:

1) Cleanup EBUSY race. On Windows, fs.rmSync without {maxRetries,retryDelay}
   races against AV scanners / file-indexers / just-exited child processes
   that still hold handles when teardown fires. Surfaces as
       EBUSY: resource busy or locked, rmdir 'C:\Users\...\AppData\Local\Temp\gsd-...'
   in bug-1736, bug-2248, bug-2698, bug-2838, and every test routing through
   the shared tests/helpers.cjs cleanup() (≈170 consumers — bug-2774 et al).
   Fix: add {maxRetries: 10, retryDelay: 100} to the shared helper plus the
   four inline cleanup sites. On POSIX the retry loop is dead code (first
   try succeeds), so no impact on ubuntu/macos.

2) bug-2839 had a local parseFrontmatter that anchored on /^---\n/ — on a
   Windows checkout with autocrlf=true the file content is CRLF, the regex
   never matches, the function returns null, and the before() hook asserts
   "agent must have YAML frontmatter" → entire suite cancels. The shared
   tests/helpers.cjs parseFrontmatter is already CRLF-aware (split /\r?\n/);
   switch to it.

Validated: plex2 (ubuntu docker) 11224/0 pass post-patch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/helpers.cjs

@@ -104,7 +104,10 @@ function createTempGitProject(prefix = 'gsd-test-') {
 }
 
 function cleanup(tmpDir) {
-  fs.rmSync(tmpDir, { recursive: true, force: true });
+  // maxRetries/retryDelay absorbs transient Windows EBUSY where AV scanners,
+  // file-indexers, or just-exited child processes still hold handles when
+  // teardown runs. On POSIX the retry loop is a no-op (rmSync succeeds first try).
+  fs.rmSync(tmpDir, { recursive: true, force: true, maxRetries: 10, retryDelay: 100 });
 }
 
 /**