Add list command to display all tracked entries

Adds Garden.List() method that returns manifest entries, unit tests
for empty and populated repos, and a CLI command that formats output
by entry type (file with hash prefix, link with target, directory).

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

garden/list.go

@@ -0,0 +1,8 @@
+package garden
+
+import "github.com/kisom/sgard/manifest"
+
+// List returns all tracked entries from the manifest.
+func (g *Garden) List() []manifest.Entry {
+	return g.manifest.Files
+}