SDK sausage-actor; examples and agent on the SDK; agent without limits

- sausage-actor: Message, messages(), send/reply/publish, async Inbox
- agent: append-only /data/history.jsonl, no caps on history, steps, waits,
  tool output, or reads; max_tokens only when configured (Anthropic: 32000)

sdk/Cargo.toml

@@ -4,4 +4,12 @@ version = "0.1.0"
 edition = "2024"
 description = "SDK for Sausage actors."
 
+[features]
+default = []
+async = ["dep:async-channel", "dep:wstd"]
+
 [dependencies]
+async-channel = { version = "2", optional = true }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+wstd = { version = "0.6", optional = true }