Move the operation envelope to huginn-mind: one spelling for client and daemon

envelope.rs (encode/decode request and response, OperationFailure,
FAILURE_SCHEMA) leaves huginn-daemon for huginn-mind, next to the wire types
it wraps. The daemon imports it and no longer re-exports it. base64 moves
with it; the daemon keeps base64 because serve.rs measures and decodes
payloads. schemars leaves the daemon, whose only user was the envelope's
OperationFailure derive. The two envelope tests run unchanged from their new
home on huginn-mind fixtures.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -477,7 +477,6 @@ dependencies = [
  "cultnet-rs",
  "huginn-mind",
  "rmp-serde",
- "schemars",
  "serde",
  "serde_json",
  "sha2",
@@ -491,6 +490,7 @@ name = "huginn-mind"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "base64",
  "chrono",
  "cultcache-rs",
  "cultnet-rs",