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.

Cargo.toml

@@ -1,9 +1,9 @@
 [workspace]
 resolver = "3"
-members = ["cli", "vfs", "interface", "sdk", "sdk/macros"]
+members = ["cli", "vfs", "interface", "sdk", "sdk/macros", "agent"]
 default-members = ["cli"]
-# The agent is rewritten on top of the new SDK. Until then it is not a member.
-exclude = ["examples", "agent"]
+# Examples are standalone plugins, each with its own empty [workspace].
+exclude = ["examples"]
 
 # Wasmtime compiles components with Cranelift. Optimize dependencies in dev
 # builds, so that tests and debug runs start plugins quickly.