Make add idempotent: skip already-tracked files instead of erroring.

Enables glob workflows like `sgard add ~/.config/mcp/services/*` to
pick up new files without failing on ones already tracked.

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

garden/garden.go

@@ -239,7 +239,7 @@ func (g *Garden) Add(paths []string, opts ...AddOptions) error {
 				// Track the directory itself as a structural entry.
 				tilded := toTildePath(abs)
 				if g.findEntry(tilded) != nil {
-					return fmt.Errorf("already tracking %s", tilded)
+					continue
 				}
 				entry := manifest.Entry{
 					Path:    tilded,
@@ -277,7 +277,7 @@ func (g *Garden) Add(paths []string, opts ...AddOptions) error {
 				}
 			}
 		} else {
-			if err := g.addEntry(abs, info, now, false, o); err != nil {
+			if err := g.addEntry(abs, info, now, true, o); err != nil {
 				return err
 			}
 		}

garden/garden_test.go

@@ -200,7 +200,7 @@ func TestAddSymlink(t *testing.T) {
 	}
 }
 
-func TestAddDuplicateRejected(t *testing.T) {
+func TestAddDuplicateIsIdempotent(t *testing.T) {
 	root := t.TempDir()
 	repoDir := filepath.Join(root, "repo")
 
@@ -218,8 +218,19 @@ func TestAddDuplicateRejected(t *testing.T) {
 		t.Fatalf("first Add: %v", err)
 	}
 
-	if err := g.Add([]string{testFile}); err == nil {
-		t.Fatal("second Add of same path should fail")
+	if err := g.Add([]string{testFile}); err != nil {
+		t.Fatalf("second Add of same path should be idempotent: %v", err)
+	}
+
+	entries := g.List()
+	count := 0
+	for _, e := range entries {
+		if e.Path == toTildePath(testFile) {
+			count++
+		}
+	}
+	if count != 1 {
+		t.Fatalf("expected 1 entry, got %d", count)
 	}
 }