YAML configuration, init without a wizard, and openai-chat-completions

- /config/agent.yaml and /config/boot.yaml replace the JSON files, read
  with serde_yaml_ng (also in the agent, inside WASM).
- sausage init asks nothing and reads no SAUSAGE_* variables. It writes
  commented templates with an example for each API, and prints the path
  to edit. Until the agent is configured, agent.chat names the missing
  field. allow_no_token: true permits a local model without a key.
- The agent speaks openai-chat-completions, the format of OpenRouter,
  Ollama, and most providers. base_url follows each SDK: with /v1 for
  the OpenAI shapes, without it for Anthropic.
- Checked against the proxy with glm-5.3-flash: the agent wrote, built,
  and activated a counter plugin in 55 s.

cli/tests/host/activation.rs

@@ -232,7 +232,7 @@ async fn review_shutdown_suppresses_hooks() {
     old["x-sausage-grace"] = json!(60);
     world.plugin("notes", old, document("serve"));
     std::fs::write(
-        world.0.join("config/boot.json"),
+        world.0.join("config/boot.yaml"),
         json!([{"method":"notes.serve","params":{}}]).to_string(),
     )
     .unwrap();
@@ -766,7 +766,7 @@ async fn restarts_service() {
     let world = TestWorld::new();
     world.plugin("notes", document("serve"), document("serve"));
     std::fs::write(
-        world.0.join("config/boot.json"),
+        world.0.join("config/boot.yaml"),
         json!([{"method":"notes.serve","params":{}}]).to_string(),
     )
     .unwrap();