feat: rewrite validation, rename built-in vars, bump python version

examples/complete.toml

@@ -5,7 +5,7 @@ tmp-dir = "/tmp/rustic"
 subsystem-name = "rustic"
 
 [settings.globalvars]
-# also built-in 
+# also built-in ara available (BASEDIR=backup-base-dir, BUILDDIR=tmp-dir)
 BTRVOL = "snap1"
 CONF_BASE = "/home/osmium/backup"
 
@@ -40,14 +40,14 @@ command = "du -sh $CONF_BASE/local-repo"
 [[hooks.after-all]]
 kind = "btrfs"
 action = "delete"
-subvolume = "$BDIR"
+subvolume = "$BASEDIR"
 
 
 [profiles.documents]
-paths = ["$BDIR/Documents"]
+paths = ["Documents"]
 
 [profiles.code]
-paths = ["$BDIR/code"]
+paths = ["code"]
 exclude-paths = [
     ".cache",
     "venv",