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
 			}
 		}