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/service.md

@@ -302,9 +302,10 @@ error types — the CLI binary converts at the assembly boundary (ADR-018).
 | Decision | ADR | Summary |
 |----------|-----|---------|
 | Assembly layer is the sole caller | [ADR-019](../../decisions/019-vault-assembly-layer-only.md) | Handlers never hold a vault reference |
+| Encryption key via HD derivation | [ADR-020](../../decisions/020-hd-derivation-for-encryption-keys.md) | Seed-derived key at `m/74'/2'/0'/0'`, not PBKDF2 |
 | RwLock for thread safety | — | Multiple readers (derive), exclusive writer (unlock/lock) |
 | TTL + LRU cache | — | Bounded memory, fresh keys, zeroized eviction |
-| Actor for in-cluster dispatch | [ADR-005](../../decisions/005-irpc-as-call-protocol-foundation.md) | irpc message dispatch; not on the call protocol |
+| Actor for in-process irpc dispatch | [ADR-005](../../decisions/005-irpc-as-call-protocol-foundation.md) | irpc message dispatch; not on the call protocol |
 | `derive_password` not cached | — | One-shot; caching grows cache with no reuse |
 
 ## Open Questions