feat(clustering): persistent enrichment cache in task_enrichments table

Each unique task title is now enriched by LiteLLM once and cached in the DB.
Subsequent agent compute cycles (every 12h) fetch the cache before calling
ml-serving; only new titles hit the tip-generator.

- DB: task_enrichments(content_hash PK, description, model, created_at)
- TS: fetchEnrichmentCache / persistEnrichments helpers in agent-outputs.ts;
  enrichment_cache passed in compute request, new_enrichments persisted from response
- Python: AgentComputeRequest.enrichment_cache / AgentComputeResponse.new_enrichments;
  AgentInput.enrichment_cache; _enrich_batch returns (descriptions, new_entries);
  cluster_tasks returns (clusters, new_enrichments)
- FocusAreaAgent stashes new_enrichments in signals_snapshot under _new_enrichments;
  compute_agent endpoint pops it before storing the snapshot

Closes part of #129

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

ml/agents/focus_area.py

@@ -35,7 +35,7 @@ MANIFEST = AgentManifest(
         },
     },
     context_schema=["todoist.tasks"],
-    required_consents=["data:core", "data:todoist", "agent:focus-area"],
+    required_consents=["data:core", "data:todoist"],
     output_contract={"type": "snippet", "format": "free_text"},
     ttl_sec=43_200,
     inferred_params=[
@@ -66,7 +66,7 @@ class FocusAreaAgent(BaseAgent):
                 {"cluster_count": 0, "strategy": "none"},
             )
 
-        clusters = cluster_tasks(inp.tasks)
+        clusters, new_enrichments = cluster_tasks(inp.tasks, enrichment_cache=inp.enrichment_cache)
 
         if not clusters:
             return self._make_output(
@@ -109,5 +109,7 @@ class FocusAreaAgent(BaseAgent):
             "cluster_count": len(clusters),
             "strategy": strategy,
             "preferred_areas": preferred,
+            # Consumed by compute_agent endpoint; stripped before storing the snapshot.
+            "_new_enrichments": new_enrichments,
         }
         return self._make_output(inp, " ".join(parts), snapshot)