feat(update): persistent state machine + lifecycle metrics

Phase 5 of v0.3. Adds an explicit, on-disk state machine to the update agent
so the lifecycle of an attempt is observable end-to-end, instead of being
inferred from logs and side effects.

New package update/pkg/state:
- Phase enum (idle, checking, downloading, staged, activated, verifying,
  success, rolled_back, failed)
- UpdateState struct persisted to /var/lib/kubesolo/update/state.json
  (overridable via --state). Atomic write (.tmp + rename). Survives reboots
  and slot switches because the file lives on the data partition.
- Transition helper that bumps AttemptCount when an attempt starts, resets
  it when the target version changes, sets/clears LastError on
  failed/success transitions, and stamps StartedAt + UpdatedAt.
- 13 unit tests cover the lifecycle, atomic write, version-change reset,
  error recording, idempotent SetFromVersion, garbage-file handling.

Wired into the existing commands:
- apply.go transitions Idle -> Checking -> Downloading -> Staged, with
  RecordError on any step failure. Reads the active slot's version file to
  populate FromVersion.
- activate.go transitions to Activated.
- healthcheck.go transitions Activated -> Verifying -> Success on pass,
  or to Failed on fail. Skips transitions if state isn't post-activation
  (manual healthcheck on a stable system shouldn't churn the state).
- rollback.go transitions to RolledBack with LastError="manual rollback".
- check.go intentionally untouched — checks are passive queries, not
  attempts; they shouldn't reset AttemptCount.

status.go gains a --json mode that emits the full state report (A/B slots,
boot counter, full UpdateState) for orchestration tooling. Human-readable
mode also prints an Update Lifecycle section when state.phase != idle.

pkg/metrics gains three new series, derived from state.json at scrape time:
- kubesolo_update_phase{phase="..."} — 1 for current, 0 for all others;
  all nine phase values always emitted so dashboards see complete series
- kubesolo_update_attempts_total
- kubesolo_update_last_attempt_timestamp_seconds
Server.SetStatePath() configures the file location; defaults to absent
which emits Idle defaults. Three new tests cover the absent / active /
all-phases-emitted cases.

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

update/cmd/metrics.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 
 	"github.com/portainer/kubesolo-os/update/pkg/metrics"
+	"github.com/portainer/kubesolo-os/update/pkg/state"
 )
 
 // Metrics starts the Prometheus-compatible metrics HTTP server.
@@ -12,10 +13,12 @@ func Metrics(args []string) error {
 	fs := flag.NewFlagSet("metrics", flag.ExitOnError)
 	listenAddr := fs.String("listen", ":9100", "Metrics HTTP listen address")
 	grubenvPath := fs.String("grubenv", "/boot/grub/grubenv", "Path to grubenv file")
+	statePath := fs.String("state", state.DefaultPath, "Path to update state.json")
 	if err := fs.Parse(args); err != nil {
 		return fmt.Errorf("parse flags: %w", err)
 	}
 
 	srv := metrics.NewServer(*listenAddr, *grubenvPath)
+	srv.SetStatePath(*statePath)
 	return srv.ListenAndServe()
 }