Delete the builder, the build asks, the build queue and the merge gate: the builder's keys are the only way a module is built (issues 1851, 1299, 1318; C2b-C5, C10)

The controller asks no build of anybody (ADR 0501 §5, ADR 0525 clause 6, ADR 0503 §3). Every module of a walk is taken from the
builder's done key; a key not done yet waits, named in the plan, and never fails it.

- C2b/C2c: internal/builder and cmd/mesh-controller's asks go (askABuild, onlyTheBuildersOwn, buildOneAsked, takeIn from an
  outcome, buildAgentsOwn, the builder's say and its fallback, the daemon hearing an outcome). The server follows the retired
  seats' built subjects only to let go of them, because the installer's carried user list names them.
- C2d: replay, queue, cancel, clear, kill, pause, resume, delivery-check, check-here and merge-gate are refused by name.
- C3: the check queues (CHECKS_READY, CHECKS_OTHER) and their workers, the checks-running and plans-building buckets and
  their grants, the build queue and its hold (plans_building.go, queue.go, link/holds.go, builds_nats.go, checks_nats.go).
- C4: merge_gate*.go, the pull request's repo-check path, the Checked event and its layers; a pull request asks the
  controller nothing but to tell mesh-config what its head declares.
- C5: the builds bucket is written only by key_takein.
- C10 (help and refusals): the terminal's help lists only what is served.

Integration tests of the plan's tiers, sends, stops and retries now run on a stand-in builder with a done key for every
module (walk_keys_scaffold_integration_test.go); the tests of the asks, the queue, the merge gate and the replays are deleted.

internal/artifacts/scratch.go

@@ -1,91 +0,0 @@
-package artifacts
-
-import (
-	"context"
-	"encoding/json"
-	"errors"
-	"fmt"
-	"io"
-	"net/http"
-	"strings"
-)
-
-// Listing and letting go of tagged documents: the check builds the scratch namespace keeps (novox/hq ADR 0350
-// §5). A check build is found by its tag, and the build seat's holder lets each go when its pull request closes
-// or it is seven days old, so the store keeps a week of check builds and no more.
-
-// Tags is every tag repository holds, in the store's order. None for a repository the store does not hold.
-func (s Store) Tags(ctx context.Context, repository string) ([]string, error) {
-	if s.Address == "" {
-		return nil, fmt.Errorf("this mesh has no artifact store on its network to list %s in", repository)
-	}
-	var tags []string
-	next := "http://" + s.Address + "/v2/" + repository + "/tags/list?n=1000"
-	for next != "" {
-		request, err := http.NewRequestWithContext(ctx, http.MethodGet, next, nil)
-		if err != nil {
-			return nil, err
-		}
-		response, err := s.client().Do(request)
-		if err != nil {
-			return nil, fmt.Errorf("cannot reach the artifact store at %s: %w", s.Address, err)
-		}
-		switch response.StatusCode {
-		case http.StatusOK:
-		case http.StatusNotFound:
-			response.Body.Close()
-			return tags, nil
-		default:
-			response.Body.Close()
-			return nil, fmt.Errorf("the artifact store answered %s for the tags of %s", response.Status, repository)
-		}
-		var page struct {
-			Tags []string `json:"tags"`
-		}
-		err = json.NewDecoder(io.LimitReader(response.Body, 8<<20)).Decode(&page)
-		link := response.Header.Get("Link")
-		response.Body.Close()
-		if err != nil {
-			return nil, fmt.Errorf("the tags of %s could not be read: %w", repository, err)
-		}
-		tags = append(tags, page.Tags...)
-		next = nextPage(link, s.Address)
-	}
-	return tags, nil
-}
-
-// nextPage is the registry's `Link: </v2/…>; rel="next"`, made whole; empty when there is no next page.
-func nextPage(link, address string) string {
-	if !strings.Contains(link, `rel="next"`) {
-		return ""
-	}
-	start, end := strings.Index(link, "<"), strings.Index(link, ">")
-	if start < 0 || end <= start+1 {
-		return ""
-	}
-	target := link[start+1 : end]
-	if strings.HasPrefix(target, "/") {
-		return "http://" + address + target
-	}
-	return target
-}
-
-// DeleteTagged lets go of the document tag names in repository: its manifest, by digest, so the next
-// collection takes its layer. Gone when the tag names nothing.
-func (s Store) DeleteTagged(ctx context.Context, repository, tag string) error {
-	if s.Address == "" {
-		return fmt.Errorf("this mesh has no artifact store on its network to let go of %s:%s in", repository, tag)
-	}
-	digest, err := s.manifestDigest(ctx, repository, tag)
-	if err != nil {
-		return err
-	}
-	if digest == "" {
-		return Gone
-	}
-	err = s.remove(ctx, s.url(repository, "manifests", digest), repository+":"+tag)
-	if errors.Is(err, Gone) {
-		return Gone
-	}
-	return err
-}