Add locked files and directory-only entries.

Locked files (--lock): repo-authoritative entries. Checkpoint skips
them (preserves repo version). Status reports "drifted" instead of
"modified". Restore always overwrites if hash differs, no prompt.
Use case: system-managed files the OS overwrites.

Directory-only entries (--dir): track directory itself without
recursing. Restore ensures directory exists with correct permissions.
Use case: directories that must exist but contents are managed
elsewhere.

Add refactored to use AddOptions struct (Encrypt, Lock, DirOnly)
instead of variadic bools.

Proto: ManifestEntry gains locked field. convert.go updated.
7 new tests. ARCHITECTURE.md and README.md updated.

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

garden/encrypt_fido2_test.go

@@ -234,7 +234,7 @@ func TestEncryptedRoundTripWithFIDO2(t *testing.T) {
 		t.Fatalf("writing: %v", err)
 	}
 
-	if err := g.Add([]string{secretFile}, true); err != nil {
+	if err := g.Add([]string{secretFile}, AddOptions{Encrypt: true}); err != nil {
 		t.Fatalf("Add: %v", err)
 	}