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.

cmd/mesh-controller/seatverbs.go

@@ -457,6 +457,28 @@ func (a *verbArguments) commandLine() ([]string, error) {
 			return nil, err
 		}
 		return []string{"forget-checks", str("owner"), str("repo"), str("number")}, nil
+	case "try":
+		// A branch tried before its pull request (novox/hq issue 686): asked, or read by the id it answered.
+		if id := str("id"); id != "" {
+			return []string{"try", "--id", id}, nil
+		}
+		if a.given["repository"] == "" && a.given["ref"] == "" {
+			// Neither shape named: the command says its usage, which names both, as `build` does.
+			return []string{"try"}, nil
+		}
+		if err := need("repository", "ref"); err != nil {
+			return nil, err
+		}
+		argv := []string{"try", str("repository"), str("ref")}
+		for _, flag := range []string{"base", "tests"} {
+			if v := str(flag); v != "" {
+				argv = append(argv, "--"+flag, v)
+			}
+		}
+		if on("store-only") {
+			argv = append(argv, "--store-only")
+		}
+		return argv, nil
 	case "cancel", "kill":
 		if err := need("id"); err != nil {
 			return nil, err
@@ -1018,6 +1040,8 @@ var jsonVerbs = map[string]bool{"status": true, "seats": true, "plan": true, "co
 	"delivery": true,
 	// The build queue, each ask with its position among the waiting (novox/hq issue 455).
 	"queue": true,
+	// A branch tried before its pull request (novox/hq issue 686).
+	"try": true,
 	// A plan followed (novox/hq issue 229): `plans wait` answers JSON; its other forms say text, and so answer no data.
 	"plans": true}
 
@@ -1308,6 +1332,10 @@ func seatToolHandlers() (map[string]link.ToolHandler, []string, error) {
 					link.AnswersWithin(ctx, bound+planWaitStart)
 				}
 			}
+			if verb == "try" && len(argv) > 1 && argv[1] == "--id" {
+				// Reading a try holds its answer for the verdict, up to its bound (novox/hq issue 686, 229).
+				link.AnswersWithin(ctx, tryWait+planWaitStart)
+			}
 			if answer, read := readHere(ctx, argv); read {
 				return answer, nil
 			}