feat(workflow): execution_plan artifact kind + schema + architect prompt

Add docs/schemas/execution_plan.json (goal/stages/edges), the
architect_freestyle prompt that instructs emitting the execution pipeline as
JSON, and register the execution_plan llm-emitted kind in both
artifacts.config.toml and sample-config.toml. Validated via
ExecutionPlanSchemaValidationTest (minimal plan passes; missing stages rejected).

examples/workflows/artifacts.config.toml

@@ -28,3 +28,8 @@ llm_emitted = true
 id = "review_report"
 schema_path = "schemas/review_report.json"
 llm_emitted = true
+
+[[artifacts]]
+id = "execution_plan"
+schema_path = "schemas/execution_plan.json"
+llm_emitted = true