0.28.0 — operational hardening (plan D1–D5): CI deploy gate, versioned images, rate limiting, resource limits, health probes

D1: deploy.sh CI gate — read-only SHA via git ls-remote, Gitea commit-status
    poll, portable mkdir deploy lock (macOS, no flock), TOCTOU guard, token
    passed via curl --config - (not argv), graceful misconfig tolerance.
D2: version-tagged images — OIKOS_VERSION=v$VERSION, keep-last-3 prune derived
    from 'docker compose config --images'; VERSION read after pull.
D3: per-IP rate limiting — new internal/httpapi/ratelimit.go (x/time/rate),
    rightmost-XFF, /healthz exempt, ctx-driven sweep; disabled by default.
D4: mem_limit/cpus on all 10 compose services.
D5: staleness-aware health probes — new internal/health package wired into
    scheduler (:8093) and notifier (:8094); nomos already had :8092.

Two /review passes hardened the deploy lock, TOCTOU guard, token hygiene,
and XFF handling.

internal/health/health_test.go

@@ -0,0 +1,73 @@
+package health
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestProbeHealthyAtBoot(t *testing.T) {
+	p := New(time.Minute)
+	if !p.Healthy() {
+		t.Fatal("probe should be healthy immediately after creation")
+	}
+}
+
+func TestProbeStaleAfterWindow(t *testing.T) {
+	p := New(50 * time.Millisecond)
+	time.Sleep(80 * time.Millisecond)
+	if p.Healthy() {
+		t.Fatal("probe should be stale after the staleness window elapses with no Bump")
+	}
+	p.Bump()
+	if !p.Healthy() {
+		t.Fatal("probe should recover immediately after Bump")
+	}
+}
+
+func TestProbeHandlerStatusCodes(t *testing.T) {
+	p := New(20 * time.Millisecond)
+
+	// Fresh → 200
+	if code := probeCode(p); code != http.StatusOK {
+		t.Fatalf("fresh probe: want 200, got %d", code)
+	}
+
+	// Stale → 503
+	time.Sleep(40 * time.Millisecond)
+	if code := probeCode(p); code != http.StatusServiceUnavailable {
+		t.Fatalf("stale probe: want 503, got %d", code)
+	}
+}
+
+func TestProbeHandlerBody(t *testing.T) {
+	p := New(time.Minute)
+	rec := httptest.NewRecorder()
+	p.Handler().ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/healthz", nil))
+
+	var body map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("invalid JSON body: %v (body=%q)", err, rec.Body.String())
+	}
+	if body["status"] != "ok" {
+		t.Fatalf("want status=ok, got %v", body["status"])
+	}
+	if _, ok := body["last_heartbeat"].(string); !ok {
+		t.Fatalf("want last_heartbeat string, got %v", body["last_heartbeat"])
+	}
+}
+
+func TestNewDefaultsStale(t *testing.T) {
+	p := New(0)
+	if p.stale <= 0 {
+		t.Fatal("New(0) should fall back to a positive staleness window")
+	}
+}
+
+func probeCode(p *Probe) int {
+	rec := httptest.NewRecorder()
+	p.Handler().ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/healthz", nil))
+	return rec.Code
+}