Point efs2.py to its canonical github repo

The client now lives at github.com/grmrgecko/qcdm-efs2; note it in the
skill so the local copy is clearly the mirror.

Claude-Session: https://claude.ai/code/session_01Ctisr9uXe4H8XsscZ2exGE

xperia-firmware-upgrade/SKILL.md

@@ -122,7 +122,8 @@ be redone.
   3. Talk EFS2 over DIAG. EfsTools (JohnBel) is the usual tool but its
      libnserial hits EIO on the option-driver port; the reliable path was a
      small pyserial EFS2 client (HDLC + FCS-16, subsys 0x4B/EFS 19). Script is
-     `5g-policyman/efs2.py` in this skill, a CLI with `ls`, `stat`, `pull`,
+     `5g-policyman/efs2.py` here; canonical copy is
+     https://github.com/grmrgecko/qcdm-efs2 . A CLI with `ls`, `stat`, `pull`,
      `push` (writes then reads back to verify) and `backup` (recursive):
      - `python3 efs2.py backup /policyman ./policyman-backup` (always first)
      - `python3 efs2.py push band_set_01.xml /policyman/band_set_01.xml`