Try a pushed branch on the build seat before its pull request (hq issue 686)

Store tests skip without a store and no agent may start one, so each failure
cost a 15-30 minute review round. The try verb asks the build seat for the
repository's merge-check.sh on the ref merged into the trunk, as a check in
the build queue, and reads its outcome from the build's log: no status on
the forge, nothing registered, nothing built for a merge.

internal/catalogue/verbs.go

@@ -362,6 +362,25 @@ 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 issue 686: a builder ran a branch's store tests for the first time in its pull request's merge check.
+	{Name: "try", Description: "Try a pushed branch or commit before its pull request (novox/hq issue 686): the build " +
+		"seat merges it into the repository's trunk and runs the repository's own merge-check.sh on the merge, in the " +
+		"toolchain containers and against the throwaway store and bus a pull request's merge check gets. Answers at " +
+		"once with an id; asked again with that id it waits up to 20 s and answers pass, fail or error, how long it " +
+		"took and the lines naming what failed — or that it is still waiting or running, to ask again. Sets no " +
+		"status on the forge, registers and builds nothing, runs no merge gate, counts as a check in the build queue " +
+		"(after deliveries) and is bounded as one. Only for a repository of the mesh; the ref must be on the forge.",
+		Input: schema(map[string]string{
+			"repository": "the repository on the forge, owner/name; with ref",
+			"ref":        "the branch or commit to try, as pushed to the forge",
+			"base":       "the branch it is merged into (the repository's default branch when none is named)",
+			"tests": "only the tests this `go test -run` pattern names, where the repository's merge-check.sh reads " +
+				"MESH_CHECK_TESTS; a script that does not runs whole",
+			"store-only": "\"true\": only the repository's store tests, where its merge-check.sh reads " +
+				"MESH_CHECK_STORE_ONLY",
+			"id": "instead of a repository: the id a try answered, to read what came of it",
+		}, nil, "store-only"),
+		Replaces: []string{"sh merge-check.sh", "docker run postgres"}},
 	// 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 " +