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>

proto/sgard/v1/sgard.proto

@@ -16,6 +16,7 @@ message ManifestEntry {
   google.protobuf.Timestamp updated  = 6;
   string plaintext_hash              = 7; // SHA-256 of plaintext (encrypted entries only)
   bool   encrypted                   = 8;
+  bool   locked                      = 9; // repo-authoritative; restore always overwrites
 }
 
 // KekSlot describes a single KEK source for unwrapping the DEK.