Step 17: Encryption core — passphrase-only, selective per-file.

Manifest schema: Entry gains Encrypted, PlaintextHash fields.
Manifest gains Encryption section with KekSlots map (passphrase slot
with Argon2id params, salt, and wrapped DEK as base64).

garden/encrypt.go: EncryptInit (generate DEK, wrap with passphrase KEK),
UnlockDEK (derive KEK, unwrap), encryptBlob/decryptBlob using
XChaCha20-Poly1305 with random 24-byte nonces.

Modified operations:
- Add: optional encrypt flag, stores encrypted blob + plaintext_hash
- Checkpoint: detects changes via plaintext_hash, re-encrypts
- Restore: decrypts encrypted blobs before writing
- Diff: decrypts stored blob before comparing
- Status: compares against plaintext_hash for encrypted entries

10 tests covering init, persistence, unlock, add-encrypted, restore
round-trip, checkpoint, status, diff, requires-DEK guard.

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

garden/mirror.go

@@ -37,7 +37,7 @@ func (g *Garden) MirrorUp(paths []string) error {
 			if lstatErr != nil {
 				return fmt.Errorf("stat %s: %w", path, lstatErr)
 			}
-			return g.addEntry(path, fi, now, true)
+			return g.addEntry(path, fi, now, true, false)
 		})
 		if err != nil {
 			return fmt.Errorf("walking directory %s: %w", abs, err)