feat(coordinator): complete tasks with an artifact_id, not a URI (CTX-05, part 3)

Task results are now coordinator-owned artifacts end to end.

- domain.Task carries ResultArtifactID instead of ResultURI/ResultSHA256;
  CompleteWith and its idempotency key are keyed on the artifact id.
- CompleteTask verifies the referenced artifact was stored for this exact
  task (rule 10): a worker cannot finish task B with task A's artifact, nor
  name an id that isn't a partial_result. Mismatch → 409.
- POST /tasks/{id}/result takes {result:{artifact_id,...}}; ListResults and
  ResultManifest follow.
- migration 0004 drops result_uri/result_sha256 and requires a completed task
  to reference its result_artifact_id.
- smoke and requests.http exercise upload → complete-by-id → replay → conflict.

coordinator/internal/usecase/dto.go

@@ -42,12 +42,11 @@ type RenewLeaseInput struct {
 }
 
 type CompleteTaskInput struct {
-	TaskID       uuid.UUID
-	WorkerID     string
-	Attempt      int
-	ResultURI    string
-	ResultSHA256 string
-	Metrics      map[string]any
+	TaskID           uuid.UUID
+	WorkerID         string
+	Attempt          int
+	ResultArtifactID uuid.UUID
+	Metrics          map[string]any
 }
 
 type UploadArtifactInput struct {