Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

internal/catalogue/verbs.go

@@ -362,6 +362,12 @@ var ControllerVerbs = []Verb{
 			"register": "\"true\" to register what it builds",
 			"older":    "\"true\", with register: even though a newer build of the module is registered",
 		}, []string{"id"}, "register", "older")},
+	// novox/hq ADR 0350 §5: asked by the delivery's owner when the forge says a pull request closed unmerged.
+	{Name: "forget-checks", Description: "Let go of every build a pull request's merge checks made for its merge in " +
+		"the artifact store's scratch namespace (novox/hq ADR 0350 §5): asked when it closed unmerged; a merged one's go " +
+		"when the walk answering its merge ends, and any check build goes after a week.",
+		Input: schema(map[string]string{"owner": "the repository's owner", "repo": "the repository",
+			"number": "the pull request's number"}, []string{"owner", "repo", "number"})},
 	{Name: "kill", Description: "End a build where it runs: the machine that took it stops its commands and containers and " +
 		"announces it failed, killed by hand — settled, never handed to another machine.",
 		Input: schema(map[string]string{"id": "the build's id"}, []string{"id"})},