Add documentation for prompt, profile, schema, and main configuration files

docs/config/config-yml.md

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+# Main `config.yml`
+
+`config.yml` defines application-level defaults used by CLI commands.
+
+By default, Scriptorium looks for `/etc/scriptorium/config.yml`. You can also pass `--config PATH`.
+
+## Complete Example
+
+```yaml
+prompt_dir: ./prompts
+profile_dir: ./profiles
+schema_dir: ./schemas
+
+server:
+  addr: :8080
+
+defaults:
+  render_format: text
+```
+
+## Available Options
+
+- `prompt_dir` (optional): Default directory for prompt definition YAML files.
+- `profile_dir` (optional): Default directory for execution profile YAML files.
+- `schema_dir` (optional): Base directory for JSON schema files used by `json_schema` validation.
+
+### `server`
+
+- `addr` (optional): HTTP server listen address for `scriptorium serve`.
+
+### `defaults`
+
+- `render_format` (optional): Default output format for `scriptorium render`.
+- Allowed values: `text`, `json`.
+
+## Precedence
+
+For run/render/serve settings, precedence is:
+
+1. Explicit CLI flags
+2. `config.yml`
+3. Built-in defaults
+
+## Notes and Rules
+
+- Unknown YAML fields fail to load (strict decoding).
+- This file does not accept API keys.
+- `config.yml` sets directory/server defaults only; prompt/profile content remains in their own files.