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

internal/snapshot/scanner.go

@@ -2018,6 +2018,14 @@ func compileExcludePatterns(patterns []string) []compiledPattern {
 		// Remove trailing slash if present (directory indicator)
 		pattern = strings.TrimSuffix(pattern, "/")
 
+		// A leading "**/" demands a "/" before the name and so misses an
+		// entry directly under the source path. An unanchored pattern
+		// already matches at every depth, so drop the prefix: as in
+		// gitignore, "**/x" then also matches a top-level "x".
+		if !anchored {
+			pattern = strings.TrimPrefix(pattern, "**/")
+		}
+
 		// Compile the glob pattern
 		// For patterns without path separators, we need to match them as components
 		// e.g., ".git" should match ".git" anywhere in the path