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/builder/check.go

@@ -103,6 +103,23 @@ type CheckSpec struct {
 	// for the walk to take. None for a group's composed check, whose members' own checks build.
 	Builds    []CheckBuildSpec
 	BuildWith *CheckBuilding
+	// Tests and StoreOnly narrow what the repository's own script runs, for a branch tried before its pull
+	// request (novox/hq issue 686): handed to the script as EnvTests and EnvStoreOnly, which it may read.
+	Tests     string
+	StoreOnly bool
+}
+
+// selection is what a check hands the repository's script to narrow what it runs (novox/hq issue 686): nothing
+// for a pull request's check, which always runs whole.
+func selection(spec CheckSpec) []string {
+	var env []string
+	if spec.Tests != "" {
+		env = append(env, EnvTests+"="+spec.Tests)
+	}
+	if spec.StoreOnly {
+		env = append(env, EnvStoreOnly+"=1")
+	}
+	return env
 }
 
 // GroupHead is one other head of a delivery group's composed check.
@@ -193,6 +210,11 @@ const (
 	// EnvGroup is a delivery group's other heads, as JSON, for the gate (novox/hq ADR 0239); empty for a
 	// pull request checked alone.
 	EnvGroup = "MESH_CHECK_GROUP"
+	// EnvTests is a `go test -run` pattern and EnvStoreOnly is 1 when a branch tried before its pull request
+	// asks for only those tests, or only the store tests (novox/hq issue 686). Never set for a pull request's
+	// check. A script that reads neither runs whole.
+	EnvTests     = "MESH_CHECK_TESTS"
+	EnvStoreOnly = "MESH_CHECK_STORE_ONLY"
 )
 
 // CheckTimeout bounds one check; a check that runs past it is an error, not a pass.
@@ -515,6 +537,7 @@ func Check(ctx context.Context, run Runner, spec CheckSpec, workspace, registry
 		EnvModules + "=" + strings.Join(append(append([]string{}, spec.Modules...), spec.New...), ","),
 		EnvGroup + "=" + groupFile,
 		"GOCACHE=" + filepath.Join(workspace, "go-cache"), "GOMODCACHE=" + filepath.Join(workspace, "go-modules")}
+	env = append(env, selection(spec)...)
 
 	v := CheckVerdict{}
 	modules := append(append([]string{}, spec.Modules...), prefixed("new:", spec.New)...)