fix(usage-log): increase lock stale window to 30 s to prevent active-lock steals

appendRecord rewrites the full token-usage.json on every write, so lock
hold time grows with file size and disk speed.  The previous stale: 5_000
was too tight: on large histories or slow disks a write can legitimately
take longer than 5 s, allowing a concurrent waiter to treat the still-
active lock as stale, reclaim it, and run an overlapping read-modify-write
cycle that silently drops the earlier writer's entry.

The risk is amplified by the attempt path where recordTokenUsage is fired
without awaiting, so multiple concurrent runs can legitimately overlap.

Fix:
• Raise stale to 30_000 ms — gives ample headroom for large files on
  slow disks while still reclaiming crashed-process locks within 30 s.
• Match the retry budget: 150 retries × 200 ms ≈ 30 s with jitter,
  so waiters exhaust retries only when the holder exceeds the stale
  window (i.e., is genuinely stuck or has crashed).

src/agents/usage-log.ts

@@ -68,14 +68,25 @@ async function readJsonArray(file: string): Promise<TokenUsageRecord[]> {
 //   • uses exponential backoff with jitter capped at stale ms
 // ---------------------------------------------------------------------------
 const APPEND_LOCK_OPTIONS: FileLockOptions = {
-  // ~100 retries × 50 ms ≈ 5 s total — matches the previous LOCK_TIMEOUT_MS.
+  // appendRecord rewrites the full token-usage.json on every call, so hold
+  // time scales with file size and disk speed.  5 s was too tight: a slow
+  // write on a large history could exceed the stale window and allow a
+  // concurrent waiter to steal an active lock, causing overlapping
+  // read-modify-write cycles that silently drop entries.
+  //
+  // 30 s gives plenty of headroom for large files on slow disks while still
+  // reclaiming locks left by crashed processes within a reasonable window.
+  // The retry budget (~150 × 200 ms = 30 s) matches the stale window so
+  // waiters exhaust retries only if the holder holds longer than stale,
+  // i.e., is genuinely stuck or crashed.
   retries: {
-    retries: 100,
+    retries: 150,
     factor: 1,
-    minTimeout: 50,
-    maxTimeout: 50,
+    minTimeout: 200,
+    maxTimeout: 200,
+    randomize: true,
   },
-  stale: 5_000,
+  stale: 30_000,
 };
 
 async function appendRecord(file: string, entry: TokenUsageRecord): Promise<void> {