Store "interrupted by restart" for jobs recovered at boot

Boot recovery now writes the required fixed text to the job error column,
and the recovery test asserts that literal text instead of the constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/jobs/jobs.go

@@ -41,7 +41,9 @@ const (
 // Failure codes the runner stores itself. A kind cannot declare them.
 const (
 	// CodeInterrupted is the error of a job a previous process left running.
-	CodeInterrupted = "interrupted"
+	// It is fixed framework text, not a snake-case code, so no kind can
+	// declare it.
+	CodeInterrupted = "interrupted by restart"
 	// CodeInternal is the error of a job whose handler failed with anything
 	// but a *Failure with a code its kind declared, or whose payload or
 	// result could not be encoded. The underlying error is neither stored
@@ -152,7 +154,7 @@ func Register[P any, R Public](r *Runner, kind string, h Handler[P, R], failureC
 		if !codePattern.MatchString(code) {
 			return nil, fmt.Errorf("jobs: register %q: failure code %q is not lower-case snake case", kind, code)
 		}
-		if code == CodeInterrupted || code == CodeInternal || code == CodeUnknownKind {
+		if code == CodeInternal || code == CodeUnknownKind {
 			return nil, fmt.Errorf("jobs: register %q: failure code %q is reserved for the runner", kind, code)
 		}
 		declared[code] = true

internal/jobs/jobs_test.go

@@ -391,8 +391,8 @@ func TestRestartFailsInterruptedJobsAndRunsQueuedOnes(t *testing.T) {
 	start(t, second)
 
 	job := waitFor(t, second, interrupted, finished)
-	if job.Status != StatusFailed || job.Error == nil || *job.Error != CodeInterrupted {
-		t.Errorf("interrupted job = %+v, want failed with %q", job, CodeInterrupted)
+	if job.Status != StatusFailed || job.Error == nil || *job.Error != "interrupted by restart" {
+		t.Errorf("interrupted job = %+v, want failed with %q", job, "interrupted by restart")
 	}
 	if string(job.Result) != `{"done":1}` {
 		t.Errorf("interrupted job result = %s, want the last partial result kept", job.Result)