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.

agent/src/main.rs

@@ -32,7 +32,7 @@ impl Agent {
     async fn chat(&self, ctx: &Ctx, p: ChatParams) -> Result<String> {
         let _guard = lock(HISTORY).await?;
         std::fs::create_dir_all("/state/agent")?;
-        let cfg: Config = serde_json::from_slice(&std::fs::read("/config/agent.json")?)?;
+        let cfg = Config::load().map_err(|error| Error::new(-32603, error))?;
         let mut entries = load_history(HISTORY)?;
         repair_tools(&mut entries)?;
         let current = entries.len();