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.

README.md

@@ -10,8 +10,9 @@ own code.
 
 ## Start
 
-You need [Nix](https://nixos.org/download) with flakes, and an API token for
-an LLM (OpenAI Responses or Anthropic Messages API). No clone needed:
+You need [Nix](https://nixos.org/download) with flakes, and an LLM: the OpenAI
+Responses API, an OpenAI-compatible Chat Completions API (OpenRouter, Ollama,
+and most other providers), or the Anthropic Messages API. No clone needed:
 
 ```sh
 nix run git+https://gitea.surma.technology/surma/sausage
@@ -27,10 +28,10 @@ nix run git+https://gitea.surma.technology/surma/sausage -- init   # creates ./w
 nix run git+https://gitea.surma.technology/surma/sausage -- run    # starts the world and a console
 ```
 
-`init` asks for the API shape (`openai-responses` or `anthropic-messages`),
-the base URL, the model, and the token. It reads `SAUSAGE_API`,
-`SAUSAGE_BASE_URL`, `SAUSAGE_MODEL`, and `SAUSAGE_API_TOKEN` first. The
-values go to `world/config/agent.json`. Both commands take another folder as
+`init` asks nothing. It writes `world/config/agent.yaml`, with a comment for
+each field and an example for each API. Edit it before the first chat: choose
+the API, the base URL, the model, and the key. Until then, a chat answers with
+an error that names the missing field. Both commands take another folder as
 an argument, for example `... -- run my-world`.
 
 Compiled code is cached outside the world, in `$XDG_CACHE_HOME/sausage/`
@@ -83,8 +84,8 @@ plugins/<name>/plugin.wasm   the current version of a plugin
 plugins/<name>/source/       its source
 state/<plugin>/              the state of a plugin, by convention
 state/core/journal.sqlite    the journal: stderr of all processes, and core events
-config/boot.json             the services that start with the world
-config/agent.json            the configuration of the agent
+config/boot.yaml             the services that start with the world
+config/agent.yaml            the configuration of the agent
 ```
 
 Each process also sees `/proc` (the processes and the state of the core),