docs(architecture): add ADR-025 — vault local-only dispatch, drop irpc

Drops irpc from alknet-vault entirely. The vault's dispatch is now direct
method calls on VaultServiceHandle — no VaultProtocol enum, no
VaultMessage, no VaultServiceActor, no mpsc channel, no Service trait, no
RemoteService trait, no postcard serialization. The vault is local-only by
construction.

The core security argument: irpc made the vault remote-capable by default
(RemoteService generated unless no_rpc is passed). The IrohProtocol handler
forwards all messages without auth. The docs framed 'register an ALPN' as a
server-setup change. This is the default-insecure anti-pattern — security
should be opt-in, not opt-out. ADR-025 inverts the default: local-only is
the only mode, and remote access requires building a separate vault-server
crate (a visible architectural act, not a flag flip).

The actor path was already dead code — service.md said 'prefer
VaultServiceHandle directly — no channel, no serialization.' The actor
existed only to make irpc's Service trait work, which existed only to make
RemoteService work, which was the footgun. VaultServiceHandle's
Arc<RwLock> provides concurrent reads and exclusive writes — better
throughput than the actor's sequential processing.

DerivedKey serialization simplifies: always redact on serialize (for
logging safety), reject '[REDACTED]' on deserialize with an error. No
'postcard preserves bytes' path. This resolves review #002 W8 (silent
corruption on JSON-deserialized DerivedKey).

Resolves:
- OQ-21: remote vault access — resolved (not deferred). Not a vault crate
  feature; if needed, a separate vault-server crate with its own ADR.
- C7: vault-server-crate question decided — not created now, not precluded.
- C8: operation access policy table dissolved — all operations local-only
  by default; if a vault-server crate exposes some remotely, that crate
  defines the policy.
- W8: DerivedKey JSON deserialization — resolved (reject redacted payloads).

Amends ADR-005 (irpc remains for alknet-call, not for alknet-vault),
ADR-018 (vault is even more standalone — zero RPC framework deps),
ADR-019 (vault is the only layer, not just the only direct-caller layer),
ADR-008 (vault integration point unchanged, but now local-only by
construction).

docs/architecture/decisions/003-crate-decomposition.md

@@ -25,7 +25,7 @@ The workspace decomposes into the following crates:
 | Crate | Responsibility | Depends on |
 |-------|---------------|------------|
 | `alknet-core` | ProtocolHandler trait, ALPN router, endpoint, BiStream, AuthContext, IdentityProvider, config, ArcSwap dynamic config | tokio, quinn, rustls, irpc, iroh (feature-gated, added by ADR-010) |
-| `alknet-vault` | Local key vault: BIP39/SLIP-0010/AES-GCM key derivation, encryption, VaultProtocol dispatch | (standalone, no alknet-core) |
+| `alknet-vault` | Local key vault: BIP39/SLIP-0010/AES-GCM key derivation, encryption | (standalone, no alknet-core) |
 | `alknet-ssh` | SshAdapter (russh, SOCKS5, port forwarding) | alknet-core, russh |
 | `alknet-call` | CallAdapter (JSON-RPC via irpc, operation registry, pub/sub, access control, call protocol client, adapter traits) | alknet-core, irpc |
 | `alknet-agent` | Agent service: LLM execution loop (forked aisdk), tool dispatch via call protocol, provider key retrieval via vault | alknet-call |