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).

docs/sample-config.toml

@@ -134,3 +134,8 @@ backend = "in_memory"          # or "turbovec"
 # The bundled examples/workflows/review_loop.toml uses this kind: the reviewer emits a
 # review_report whose `verdict` field gates the implementer↔reviewer loop via
 # artifact_field_equals transitions. See docs/schemas/review_report.json for the schema.
+
+[[artifacts]]
+id = "execution_plan"
+schema_path = "schemas/execution_plan.json"
+llm_emitted = true