fix: adversarial-review runtime fixes across the telemetry + kill paths

- session-runner: exit-labeling keys off 'exit', not 'close' — an orphan
  holding the pipes could relabel a REAL exit (auth failure) as
  'timeout_startup' availability noise; the kill path still always
  group-kills and cancels the reader (labeling and unblocking are separate
  concerns). Work phase arms on a flag, not firstResponseMs===0 (a same-ms
  first byte left the startup timer live all run). The CI startup grace is
  now a real FLOOR (Math.max), matching its name and pinning test.
- gstack-detach: pgid captured AT SPAWN (== child pid under
  start_new_session) — resolving it after the grace raised ESRCH once the
  leader died on SIGTERM, orphaning TERM-immune grandchildren forever.
- test-free-shards: ledger entries carry branch + git_sha (rev-parse split:
  '--abbrev-ref HEAD HEAD' printed the branch twice and recorded it as the
  sha); local ledger default is per-PROJECT, not the machine-global tmpdir.
- eval-flake-rank: per-LINE ledger parse (one torn JSONL line vanished the
  whole series), 60-day recency bound (transcript-bearing files are MBs),
  shared isFinalizedEvalResultFile predicate (the artifact-taxonomy rule
  lived in three places); eval-store exports the predicate and finalize
  stops computing flakyRetries twice; paid-shards cleanup uses async rm
  (a SIGKILLed shard's git-workspace teardown blocked every sibling's
  stream classification on the parent event loop).

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

test/helpers/eval-store.ts

@@ -192,6 +192,21 @@ export function isPartialEval(data: unknown, filename: string): boolean {
   return Boolean((data as { _partial?: unknown } | null)?._partial);
 }
 
+/**
+ * Is this path a FINALIZED eval-store result file? Single owner of the
+ * filename taxonomy (manifest.json / slice-N.json are runner artifacts,
+ * _partial* are in-progress accumulators) — the paid runner's report mode
+ * and eval-flake-rank both consume this instead of re-encoding the rule
+ * (review finding: the rule lived in three places).
+ */
+export function isFinalizedEvalResultFile(relPath: string): boolean {
+  const base = path.basename(relPath);
+  if (!base.endsWith('.json')) return false;
+  if (base === 'manifest.json' || /^slice-\d+\.json$/.test(base)) return false;
+  if (base.startsWith('_partial')) return false;
+  return true;
+}
+
 /**
  * List eval JSON files in `evalDir` plus one level of `<evalDir>/shards/<slug>/`
  * subdirectories (where the sharded paid runner points each shard's collector).
@@ -900,6 +915,7 @@ export class EvalCollector {
     const totalDuration = this.tests.reduce((s, t) => s + t.duration_ms, 0);
     const passed = this.tests.filter(t => t.passed).length;
 
+    const flaky = this.flakyRetries();
     const result: EvalResult = {
       schema_version: SCHEMA_VERSION,
       version,
@@ -917,7 +933,7 @@ export class EvalCollector {
       wall_clock_ms: Date.now() - this.createdAt,
       tests: this.tests,
       ...(this.shard ? { shard: this.shard } : {}),
-      ...(this.flakyRetries().length > 0 ? { flaky_retries: this.flakyRetries() } : {}),
+      ...(flaky.length > 0 ? { flaky_retries: flaky } : {}),
     };
 
     // Write eval file