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.

DECISIONS.md

@@ -61,6 +61,9 @@ a plugin, or a change that the agent makes to itself.
 
 ## Small choices
 
+- **YAML for configuration, JSON for data.** A person edits configuration, so
+  it needs comments. `init` writes commented templates instead of asking
+  questions, so there is one way to configure a world.
 - **Plain POSIX files.** Programs like `tempfile`, git, and SQLite expect
   normal files. Atomic writes use a temporary file and `rename`.
 - **Tests run under WASI.** `cargo test` on the host cannot find failures