agent: a plugin on the new SDK (agent.chat, tools call/read_file/write_file/list_dir/copy_dir)

Holds the history lock for a turn, 600 s grace period, interim texts as
core.notify. Tests with a fake LLM in the real core. The SDK tests of
/proc, /self, /run, and detached notifications now run.

agent/Cargo.toml

@@ -2,10 +2,12 @@
 name = "agent"
 version = "0.1.0"
 edition = "2024"
-description = "The Sausage agent actor."
+description = "The Sausage agent plugin."
 
 [dependencies]
-sausage-actor = { version = "0.1", path = "../sdk", features = ["async"] }
+sausage-plugin = { version = "0.1", path = "../sdk" }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+
+[target.'cfg(target_os = "wasi")'.dependencies]
 wstd = "0.6"