docs(architecture): add ADR-020, resolve OQ-20 — HD derivation for encryption keys

The vault uses SLIP-0010 HD derivation from the BIP39 seed for the
AES-256-GCM encryption key, not PBKDF2. This replaces the TypeScript
predecessor's (@alkdev/storage/src/graphs/crypto.ts) PBKDF2-based
approach.

Key decisions:
- HD derivation at m/74'/2'/0'/0' produces the encryption key
- PBKDF2 is not implemented in the vault; no password-based derivation
- salt field is unused in v2 (wire-format compat only)
- key_version=1 reserved for TS PBKDF2 data; key_version=2 for vault HD
- TS-encrypted data requires one-time migration to v2
- CURRENT_KEY_VERSION changes from 1 to 2 (source drift flagged)

OQ-20 resolved: the encryption key derivation method is locked. OQ-22
(key rotation workflow) remains open but does not block implementation.

docs/architecture/crates/vault/mnemonic-derivation.md

@@ -283,8 +283,9 @@ assembly-layer concern.
 
 See [open-questions.md](../../open-questions.md) for full details.
 
-- **OQ-20** (open): Salt/KDF Phase B — the `EncryptedData.salt` field is
-  reserved; v1 does not use it. See [encryption.md](encryption.md).
+- **OQ-20** (resolved by ADR-020): Encryption key derivation — HD derivation
+  from seed, not PBKDF2. The salt field is unused in v2. See
+  [encryption.md](encryption.md).
 
 ## References