fix: auto-clear stale heartbeat when process is dead

Add PID to heartbeat file. eval-watch checks process.kill(pid, 0) and
auto-deletes the heartbeat when the PID is no longer alive — no manual
cleanup needed after crashed/killed E2E runs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/helpers/session-runner.ts

@@ -216,6 +216,7 @@ export async function runSkillTest(options: {
                     const toolDesc = `${item.name}(${truncate(JSON.stringify(item.input || {}), 60)})`;
                     atomicWriteSync(HEARTBEAT_PATH, JSON.stringify({
                       runId,
+                      pid: proc.pid,
                       startedAt,
                       currentTest: testName,
                       status: 'running',