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/config/config.go

@@ -35,6 +35,13 @@ type Config struct {
 	Channel           string
 	MaintenanceWindow string
 	PubKey            string
+	// HealthcheckURL is an optional URL the healthcheck command will GET;
+	// 200 = pass, anything else = fail.
+	HealthcheckURL string
+	// AutoRollbackAfter is the number of consecutive post-boot healthcheck
+	// failures after which the agent will call Rollback automatically.
+	// 0 = disabled (default).
+	AutoRollbackAfter int
 }
 
 // Load reads and parses update.conf. A missing file returns an empty Config
@@ -73,6 +80,21 @@ func Load(path string) (*Config, error) {
 			c.MaintenanceWindow = value
 		case "pubkey":
 			c.PubKey = value
+		case "healthcheck_url":
+			c.HealthcheckURL = value
+		case "auto_rollback_after":
+			// Parse a small integer. Non-numeric values are silently
+			// ignored (forward compat); zero disables the feature.
+			n := 0
+			for _, ch := range value {
+				if ch >= '0' && ch <= '9' {
+					n = n*10 + int(ch-'0')
+				} else {
+					n = 0
+					break
+				}
+			}
+			c.AutoRollbackAfter = n
 		}
 		// Unknown keys are silently ignored for forward compatibility.
 	}