Implement artifact-level render command with Markdown output and update docs

docs/config.md

@@ -23,6 +23,17 @@ For `trim`:
 - If `--output-schema` is omitted, output preserves the input artifact schema.
 - If `--output-schema` is set, it must be one of `seriatim-minimal`, `seriatim-intermediate`, `seriatim-full`.
 
+## Render format and defaults
+
+`render` requires `--format`. Current supported value is `markdown`.
+
+Render defaults:
+
+- `--title`: `Transcript`
+- `--include-timestamps`: `true`
+- `--include-segment-ids`: `false`
+- `--include-metadata`: `false`
+
 ## Merge module defaults
 
 Default merge module selections:
@@ -152,6 +163,11 @@ All commands:
 - Validates `--output-schema` through the same schema set as `merge`.
 - Currently accepts only `json` in `--output-modules`.
 
+`render`:
+
+- Requires `--input-file`, `--output-file`, and `--format`.
+- Validates `--format` as `markdown`.
+
 ## Related docs
 
 - CLI reference: [cli.md](cli.md)