docs(architecture): remove derive_password and site_password_path from vault

The password-manager pattern (deterministic per-site passwords from HD
derivation) is not relevant to an RPC system's vault. Handlers call APIs
(using API keys, OAuth tokens, mTLS), not websites with passwords. The
vault is for cryptographic key derivation and credential encryption.

Removes:
- derive_password, derive_password_string from service.md
- site_password_path from mnemonic-derivation.md
- m/74'/1'/0'/{hash}' path from PATHS module and path semantics table
- derive_password row from the cache table

Resolves review #002 C9 (site_password_path hash mapping underspecified)
by removing the feature rather than specifying the non-standard
string→u32 mapping and Ed25519-as-password-entropy construction.

If deterministic password generation is ever needed (browser-automation
edge case), it can be re-added — the cost is near-zero. Removing it now
eliminates permanent API surface inherited from a prior project's
password-manager pattern.

docs/architecture/decisions/025-vault-local-only-dispatch.md

@@ -203,6 +203,19 @@ version of ADR-018's intent.
   exclusive writes (unlock/lock). The actor's sequential processing was
   actually *worse* for throughput than the RwLock. Removing the actor
   makes the concurrency model visible and correct.
+- `derive_password` and `site_password_path` are removed from the vault's
+  API and path model. The password-manager pattern (deterministic per-site
+  passwords from HD derivation) is not relevant to an RPC system's vault —
+  handlers call APIs (using API keys, OAuth tokens, mTLS), not websites
+  with passwords. The vault is for cryptographic key derivation and
+  credential encryption. This resolves review #002 C9 (site_password_path
+  hash mapping underspecified) by removing the feature rather than
+  specifying the non-standard string→u32 mapping and Ed25519-as-password-
+  entropy construction. If deterministic password generation is ever needed
+  (browser-automation edge case), it can be re-added or implemented as a
+  separate concern — the cost is near-zero, and removing it now eliminates
+  permanent API surface that was inherited from a prior project's
+  password-manager pattern.
 
 **Negative:**
 
@@ -247,6 +260,11 @@ version of ADR-018's intent.
   in protocol.md goes away. If a future vault-server crate exposes some
   operations remotely, *that crate* defines the access policy in its own
   ADR.
+- **C9 (site_password_path hash mapping underspecified)**: resolved. The
+  `derive_password` / `derive_password_string` / `site_password_path`
+  methods are removed from the vault's API. The password-manager pattern
+  is not relevant to an RPC system's vault. No hash mapping to specify,
+  no Ed25519-as-password-entropy question to answer.
 
 ## Assumptions