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/Cargo.toml

@@ -8,6 +8,7 @@ description = "The Sausage agent plugin."
 sausage-plugin = { version = "0.1", path = "../sdk" }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+serde_yaml_ng = "0.10"
 
 [target.'cfg(target_os = "wasi")'.dependencies]
 wstd = "0.6"