Make a leading **/ in an exclude pattern match at the top level (closes #303)

An unanchored exclude pattern such as **/node_modules or **/*.pyc
needed a / before the name, so it excluded sub/node_modules and
sub/y.pyc but backed up node_modules/ and x.pyc directly under the
snapshot path. An unanchored pattern already matches at every depth,
so the leading **/ is now dropped when the pattern is compiled and,
as in gitignore, the pattern matches the top level too.

Judgement call: an anchored /**/x is left as it was; the issue covers
patterns that start with **/.

Model: opus-5-5

TODO.md

@@ -22,6 +22,15 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-13: Made an exclude pattern starting with `**/` also match
+  directly under the snapshot path
+  ([issue #303](https://git.eeqj.de/sneak/vaultik/issues/303)).
+  `**/node_modules` and `**/*.pyc`, the patterns `config.example.yml`
+  uses, excluded `sub/node_modules` and `sub/y.pyc` but backed up
+  `node_modules/` and `x.pyc` at the top of the path, since the pattern
+  required a `/` before the name. As in gitignore, such a pattern now
+  matches at every depth, the top included.
+
 - 2026-10-12: Rejected a retention duration longer than about 292 years
   ([issue #289](https://git.eeqj.de/sneak/vaultik/issues/289)). The
   number given to `--older-than` or `--keep-newer-than` was multiplied