fix(ci): cap the ratchet's vitest worker count to 2

#4307: Test Suite (ubuntu-latest) intermittently crashes with tinypool's
ERR_IPC_CHANNEL_CLOSED at three different, unrelated points across 7 runs
on this PR, after ruling out any specific file as the cause (one
candidate, pod-tick.test.mjs, was fixed separately for an unrelated
reason and the crash still recurred at a different file). The pattern
(crash location varies, happens ~60-90s into a ~9500-test run) is more
consistent with cumulative memory pressure across the default fork
pool (one fork per CPU, ~4 on a standard runner) than a single bad
test. Capping workers at 2 trades some wall-clock time for headroom
while the real cause is still open in #4307.

Co-Authored-By: RuFlo <ruv@ruv.net>

scripts/ci-test-ratchet.mjs

@@ -128,6 +128,14 @@ function main() {
     const run = spawnSync(process.execPath, [
       vitestBin,
       'run',
+      // #4307: the default fork pool (one fork per CPU, ~4 on a standard
+      // GitHub-hosted runner) intermittently crashes with tinypool's
+      // ERR_IPC_CHANNEL_CLOSED partway through this ~9500-test suite — observed
+      // at three different, unrelated points across repeated runs, consistent
+      // with cumulative memory pressure rather than one bad file. Capping forks
+      // trades wall-clock time for headroom; raise this back (or drop it) if a
+      // future profiling pass finds the real cause and fixes it properly.
+      '--maxWorkers=2',
       // The new guidance file uses Claude Code's native test kit, not
       // Vitest. Do not enlarge the historical known-failure baseline.
       '--exclude=plugins/ruflo-mods/tests/guidance.test.ts',