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

docs/policy/development.md

@@ -16,6 +16,7 @@ It complements [architecture policy](architecture.md) and
 - `internal/artifact/`: conversion from internal merged model to public shapes.
 - `internal/trim/`: artifact-level trim logic.
 - `internal/normalize/`: artifact-level normalize parsing/building.
+- `internal/render/`: artifact-level rendering and renderer registry.
 - `internal/*` domain packages: overlap, coalesce, danglers, filler,
   backchannel, speaker, autocorrect, report, model.
 - `schema/`: public structs plus embedded JSON Schemas and validation.
@@ -36,6 +37,7 @@ go run ./cmd/seriatim --help
 go run ./cmd/seriatim merge --help
 go run ./cmd/seriatim trim --help
 go run ./cmd/seriatim normalize --help
+go run ./cmd/seriatim render --help
 ```
 
 Current toolchain note: