step 5 complete: execution plan builder, template registry, and LRU plan cache

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

BACKEND_PLAN.md

@@ -181,8 +181,8 @@ adiuva-api/
 - [x] Integration tests with mocked LLM and mocked agents
 - **Outcome:** Intelligent routing with single-agent and pipeline modes.
 
-### Step 5 — Execution Plan generator
-- [ ] `app/core/execution_plan.py`:
+### Step 5 — Execution Plan generator ✅
+- [x] `app/core/execution_plan.py`:
   - `PromptTemplateRegistry`: dict of `template_id -> prompt_text`. Templates are server-side only — client receives IDs.
   - `ExecutionPlanBuilder`:
     - `add_step(action, params) -> self`