chore: prep Phase 3 tasks and workspace for alknet-secret development

- Add irpc (0.16) and irpc-derive (0.16) as workspace dependencies
- Add irpc, irpc-derive, and secp256k1 (optional) to alknet-secret Cargo.toml
- Clarify encryption-salt-kdf task: Option B (document salt as reserved) is the
  chosen path per spec update, removing Option A acceptance criteria
- Update irpc-secret-protocol-integration task with concrete irpc crate details:
  real crate on crates.io v0.16, #[rpc_requests] macro, workspace config,
  AuthProtocol pattern reference, DerivedKey serialization considerations
- Fix secp256k1-ethereum-derivation task: correct crate name is secp256k1
  (not libsecp256k1), add version pin 0.29

Cargo.toml

@@ -11,4 +11,8 @@ resolver = "2"
 version = "0.1.0"
 edition = "2021"
 license = "MIT OR Apache-2.0"
-repository = "https://git.alk.dev/alkdev/alknet"
+repository = "https://git.alk.dev/alkdev/alknet"
+
+[workspace.dependencies]
+irpc = "0.16"
+irpc-derive = "0.16"