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/cmd/coordinator/main.go

@@ -72,7 +72,7 @@ func run() error {
 		CreateJob:        usecase.NewCreateJob(jobRepo, taskRepo, tx, clk),
 		ClaimTask:        usecase.NewClaimTask(taskRepo, clk, cfg.LeaseDuration),
 		RenewLease:       usecase.NewRenewLease(taskRepo, tx, clk, cfg.LeaseDuration),
-		CompleteTask:     usecase.NewCompleteTask(taskRepo, jobRepo, tx, clk),
+		CompleteTask:     usecase.NewCompleteTask(taskRepo, jobRepo, artifactRepo, tx, clk),
 		FailTask:         usecase.NewFailTask(taskRepo, jobRepo, tx, clk),
 		GetJobStatus:     usecase.NewGetJobStatus(jobRepo, taskRepo),
 		UploadArtifact:   usecase.NewUploadArtifact(taskRepo, artifactRepo, blobStore, clk),