Fix run_id format to be filename-safe

Remove colons from timestamp format to avoid 'Invalid argument' errors
when creating log files. Changes from ISO format (2026-02-05T00:21:57Z)
to compact format (20260205T002157Z).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: fluidnumerics-joe

src/xfer/cli.py

@@ -54,7 +54,8 @@
 # Helpers
 # -----------------------------
 def now_run_id() -> str:
-    ts = dt.datetime.utcnow().replace(microsecond=0).isoformat() + "Z"
+    # Use filename-safe format (no colons)
+    ts = dt.datetime.utcnow().strftime("%Y%m%dT%H%M%SZ")
     rnd = os.urandom(3).hex()
     return f"{ts}_{rnd}"