Add file exclusion support (sgard exclude/include).

Paths added to the manifest's exclude list are skipped during Add,
MirrorUp, and MirrorDown directory walks. Excluding a directory
excludes everything under it. Already-tracked entries matching a
new exclusion are removed from the manifest.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

garden/garden.go

@@ -256,6 +256,13 @@ func (g *Garden) Add(paths []string, opts ...AddOptions) error {
 					if err != nil {
 						return err
 					}
+					tilded := toTildePath(path)
+					if g.manifest.IsExcluded(tilded) {
+						if d.IsDir() {
+							return filepath.SkipDir
+						}
+						return nil
+					}
 					if d.IsDir() {
 						return nil
 					}