Register a built-in noop job kind and store only failure codes

The server now registers the framework's built-in noop kind through
registerJobKinds before the runner starts, so production can run a job; a
noop job succeeds with {"ok":true}. Each feature's kinds are registered in
the same function.

Handlers report failure only with a *jobs.Failure code their kind declared
at registration. The runner stores and returns only that code, or
internal_error, interrupted or unknown_kind, and logs only the job id, kind,
code and a fixed reason: handler error text is never stored, returned or
logged. Results implement jobs.Public, whose PublicJSON names exactly the
data that is stored and returned.

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

internal/jobs/builtin.go

@@ -0,0 +1,44 @@
+package jobs
+
+import (
+	"context"
+	"encoding/json"
+)
+
+// KindNoop is the framework's built-in job kind: it does no work and
+// succeeds with {"ok":true}. It is the runner's own trivial kind, as Feature
+// 09's trivial test skill is the harness's, and the startup and readiness
+// job: a noop job reaching succeeded proves the runner claims, runs and
+// records jobs.
+const KindNoop = "noop"
+
+// NoopPayload is the payload of a noop job.
+type NoopPayload struct{}
+
+// NoopResult is the result of a noop job.
+type NoopResult struct{}
+
+// PublicJSON returns {"ok":true}.
+func (NoopResult) PublicJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		OK bool `json:"ok"`
+	}{OK: true})
+}
+
+func runNoop(context.Context, NoopPayload, func(NoopResult) error) (NoopResult, error) {
+	return NoopResult{}, nil
+}
+
+// Builtins enqueues the framework's built-in job kinds.
+type Builtins struct {
+	Noop *Kind[NoopPayload]
+}
+
+// RegisterBuiltins registers the framework's built-in job kinds on r.
+func RegisterBuiltins(r *Runner) (*Builtins, error) {
+	noop, err := Register(r, KindNoop, runNoop)
+	if err != nil {
+		return nil, err
+	}
+	return &Builtins{Noop: noop}, nil
+}