feat(update): pre-flight gates + deeper healthcheck + auto-rollback

Phase 8 of v0.3. Tightens the update lifecycle on both ends.

Pre-flight (apply.go, before any download):
- Free-space check on the passive partition: image size + 10% headroom must
  be available. Uses statfs(2) via the new pkg/partition.FreeBytes /
  HasFreeSpaceFor helpers (tests cover happy path, tiny request, huge
  request, missing path). Catches corrupted-FS and shrunk-partition cases
  before we destroy the existing slot data.
- Node-block-label check: refuses if the local K8s node carries the
  updates.kubesolo.io/block=true label. New pkg/health.CheckNodeBlocked
  shells out to kubectl per the project's zero-deps stance. Silently bypassed
  when no kubeconfig is reachable (air-gap case). Skipped by --force.

Healthcheck (extended via new pkg/health/extended.go + preflight.go):
- CheckKubeSystemReady waits until every kube-system pod has held the Running
  phase for >= N seconds (default 30). Catches "started ok, will crash-loop"
  bugs that a single-shot phase check misses.
- CheckProbeURL fetches an operator-supplied URL; 200 = pass. Wired through
  update.conf as healthcheck_url= and cloud-init updates.healthcheck_url.
- CheckDiskWritable writes/fsyncs/reads a 1-KiB probe under /var/lib/kubesolo.
  Always runs in healthcheck so a wedged data partition fails fast.
- pkg/health.Status grows KubeSystemReady, ProbeURL, DiskWritable booleans.
  Optional checks default to true in RunAll() so they don't block when
  unconfigured. health_test.go updated to the new 6-field shape.

Auto-rollback (healthcheck.go):
- state.UpdateState gains HealthCheckFailures (consecutive post-Activated
  failures). Reset on a clean pass.
- --auto-rollback-after N (also auto_rollback_after= in update.conf) triggers
  env.ForceRollback() when the failure count reaches the threshold. State
  transitions to RolledBack with a descriptive LastError. The command still
  exits with the healthcheck error; the operator/init is expected to reboot.
- Only fires while Phase == Activated. Doesn't second-guess a long-stable
  system that happens to fail one healthcheck.

config / opts / cloud-init plumbing:
- update.conf gains healthcheck_url= and auto_rollback_after= keys.
- New CLI flags: --healthcheck-url, --auto-rollback-after, --kube-system-settle.
- cloud-init full-config.yaml documents the new updates: subfields.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

update/pkg/health/extended_test.go

@@ -0,0 +1,77 @@
+package health
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestCheckProbeURLEmptyAlwaysPasses(t *testing.T) {
+	ok, err := CheckProbeURL("")
+	if err != nil {
+		t.Fatalf("CheckProbeURL(\"\"): %v", err)
+	}
+	if !ok {
+		t.Error("empty probe URL should return ok=true (check disabled)")
+	}
+}
+
+func TestCheckProbeURL200(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+	ok, err := CheckProbeURL(srv.URL)
+	if err != nil {
+		t.Fatalf("CheckProbeURL: %v", err)
+	}
+	if !ok {
+		t.Error("expected ok=true on 200")
+	}
+}
+
+func TestCheckProbeURLNon200(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusServiceUnavailable)
+	}))
+	defer srv.Close()
+	ok, err := CheckProbeURL(srv.URL)
+	if err != nil {
+		t.Fatalf("CheckProbeURL: %v", err)
+	}
+	if ok {
+		t.Error("expected ok=false on 503")
+	}
+}
+
+func TestCheckProbeURLNetworkError(t *testing.T) {
+	// Port 1 is reserved (tcpmux) and never bound by Linux defaults.
+	_, err := CheckProbeURL("http://127.0.0.1:1")
+	if err == nil {
+		t.Error("expected error for unreachable URL, got nil")
+	}
+}
+
+func TestCheckDiskWritableHappyPath(t *testing.T) {
+	dir := t.TempDir()
+	ok, err := CheckDiskWritable(dir)
+	if err != nil {
+		t.Fatalf("CheckDiskWritable: %v", err)
+	}
+	if !ok {
+		t.Error("expected ok=true on writable temp dir")
+	}
+	// Probe file should have been cleaned up.
+	if _, err := os.Stat(filepath.Join(dir, ".update-probe")); !os.IsNotExist(err) {
+		t.Errorf("probe file not cleaned up: stat err=%v", err)
+	}
+}
+
+func TestCheckDiskWritableMissingDir(t *testing.T) {
+	_, err := CheckDiskWritable("/this/path/does/not/exist")
+	if err == nil {
+		t.Error("expected error for missing dataDir, got nil")
+	}
+}