Add Job Recipe API spec and design doc

Add docs/design/job_recipe_spec.md covering the Recipe API interface design:
- User guide (constructor, export/execute, filters, config, decomposers, per-site config, model/checkpoint)
- Execution environments (SimEnv, PocEnv, ProdEnv)
- Memory management (server_memory_gc_rounds with per-recipe defaults)
- 14 API enhancement requirements (deploy map, multi-script/launcher, k8s, algorithm config, optional integrations, resource/GPU, client tracking, job scope, client memory)
- Proposed API changes for each requirement with recommendations

Author: chesterxgchen

docs/design/README.md

@@ -0,0 +1,3 @@
+# Designs
+
+Technical design and architecture documents.