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.

flake.nix

@@ -126,7 +126,7 @@
           emptyPath="$root/empty-bin"
           mkdir -p "$emptyPath" "$root/home" "$root/cargo-home"
           mkdir -p "$world/plugins" "$world/state" "$world/config"
-          echo "[]" > "$world/config/boot.json"
+          echo "[]" > "$world/config/boot.yaml"
           source="$world/plugins/probe/source"
           mkdir -p "$source/src" "$source/tests"
           cat > "$source/Cargo.toml" <<'TOML'