Snapper: no question on btrfs; vendor the SysGuides scripts

- On btrfs, snapshots + zstd compression now just happen (no prompt);
  --snapper=no remains the explicit opt-out.
- Vendor scripts/snapper-*.sh + config/snapper.actions from
  SysGuides/sysguides-snapper-fedora (MIT, upstream 0a28eb4) into
  vendor/snapper/ and install from there — no runtime clone of an
  external repo, and no dependency on exec bits (fixes the
  './install.sh: Permission denied' failure seen on the test VM).
- grub-btrfs still cloned at runtime from Antynea/grub-btrfs with the
  Fedora config applied; everything else is local now.

vendor/snapper/README.md

@@ -0,0 +1,11 @@
+# Vendored: SysGuides snapper scripts for Fedora
+
+Source: https://github.com/SysGuides/sysguides-snapper-fedora (MIT, Madhu Desai / SysGuides)
+
+Vendored (2026-10-27, upstream commit 0a28eb4) so setup does not depend on an
+external repo at runtime. `lib/common.sh:setup_snapper` installs:
+
+- scripts/snapper-*.sh  -> /usr/local/bin/ (DNF5 pre/post snapshot actions)
+- config/snapper.actions -> /etc/dnf/libdnf5-plugins/actions.d/
+
+To update: re-fetch from upstream, diff, commit here.