fix(compression-lock): reclaim crashed holders instead of stranding the lease

The compression lock was gated so that any holder on a Windows host was
assumed alive until the full TTL expired. A crashed holder therefore
stranded every other agent on the session for the whole lease window.

Probe liveness with psutil.pid_exists, which is safe on nt. Keep the
os.kill(pid, 0) path strictly for POSIX: on Windows signal 0 maps to
CTRL_C_EVENT (bpo-14484) and can kill the target's console group, so with
psutil absent the only safe answer stays 'assume alive' and let the TTL
run out.

Also carry the commit fence through cancelled compressions so an aborted
run leaves the lock reacquirable rather than half-held.

agent/conversation_compression.py

@@ -671,7 +671,17 @@ class _CompressionLockLeaseRefresher:
         # by the TTL the acquirer set — the lock can never be held past its TTL
         # by a stuck refresher.
         consecutive_failures = 0
-        while not self._stop.wait(self._refresh_interval_seconds):
+        # First refresh happens immediately, not one interval late. Everything
+        # between try_acquire() and start() (the rotation-ownership lookup, the
+        # durable-breaker re-read, thread startup) is charged against the very
+        # first lease, so on a short TTL under load the lock could already be
+        # expired — and reclaimable by a competing path — before tick #1.
+        first = True
+        while first or not self._stop.wait(self._refresh_interval_seconds):
+            if first:
+                first = False
+                if self._stop.is_set():
+                    break
             try:
                 refreshed = self._db.refresh_compression_lock(
                     self._session_id,