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/exclude.go

@@ -0,0 +1,80 @@
+package garden
+
+import (
+	"fmt"
+	"path/filepath"
+)
+
+// Exclude adds the given paths to the manifest's exclusion list. Excluded
+// paths are skipped during Add and MirrorUp directory walks. If any of the
+// paths are already tracked, they are removed from the manifest.
+func (g *Garden) Exclude(paths []string) error {
+	existing := make(map[string]bool, len(g.manifest.Exclude))
+	for _, e := range g.manifest.Exclude {
+		existing[e] = true
+	}
+
+	for _, p := range paths {
+		abs, err := filepath.Abs(p)
+		if err != nil {
+			return fmt.Errorf("resolving path %s: %w", p, err)
+		}
+
+		tilded := toTildePath(abs)
+
+		if existing[tilded] {
+			continue
+		}
+
+		g.manifest.Exclude = append(g.manifest.Exclude, tilded)
+		existing[tilded] = true
+
+		// Remove any already-tracked entries that match this exclusion.
+		g.removeExcludedEntries(tilded)
+	}
+
+	if err := g.manifest.Save(g.manifestPath); err != nil {
+		return fmt.Errorf("saving manifest: %w", err)
+	}
+
+	return nil
+}
+
+// Include removes the given paths from the manifest's exclusion list,
+// allowing them to be tracked again.
+func (g *Garden) Include(paths []string) error {
+	remove := make(map[string]bool, len(paths))
+	for _, p := range paths {
+		abs, err := filepath.Abs(p)
+		if err != nil {
+			return fmt.Errorf("resolving path %s: %w", p, err)
+		}
+		remove[toTildePath(abs)] = true
+	}
+
+	filtered := g.manifest.Exclude[:0]
+	for _, e := range g.manifest.Exclude {
+		if !remove[e] {
+			filtered = append(filtered, e)
+		}
+	}
+	g.manifest.Exclude = filtered
+
+	if err := g.manifest.Save(g.manifestPath); err != nil {
+		return fmt.Errorf("saving manifest: %w", err)
+	}
+
+	return nil
+}
+
+// removeExcludedEntries drops manifest entries that match the given
+// exclusion path (exact match or under an excluded directory).
+func (g *Garden) removeExcludedEntries(tildePath string) {
+	kept := g.manifest.Files[:0]
+	for _, e := range g.manifest.Files {
+		if !g.manifest.IsExcluded(e.Path) {
+			kept = append(kept, e)
+		}
+	}
+	g.manifest.Files = kept
+}