feat(update): channels, maintenance windows, min-version gate

Phase 6 of v0.3. The update agent now refuses to apply artifacts whose
channel doesn't match local policy, whose architecture differs from the
running host, or whose min_compatible_version is above the current
version. It also refuses to apply outside a configured maintenance window
unless --force is given.

New package update/pkg/config:
- config.Load parses /etc/kubesolo/update.conf (key=value, # comments,
  unknown keys ignored). Missing file is fine — fresh systems before
  cloud-init has run.
- ParseWindow handles "HH:MM-HH:MM" plus the wrapping midnight case
  (e.g. "23:00-01:00"). Empty input -> AlwaysOpen (no constraint).
  Degenerate zero-length windows never match.
- CompareVersions does a simple 3-component semver compare with the 'v'
  prefix optional and pre-release suffix ignored.
- 14 unit tests total.

update/pkg/image/image.UpdateMetadata gains three optional fields:
- channel ("stable", "beta", ...)
- min_compatible_version (refuse upgrade if current < this)
- architecture ("amd64", "arm64", ...)

update/cmd/opts.go reads update.conf and merges it into opts; explicit
--server / --channel / --pubkey / --maintenance-window CLI flags override
the file. New --force, --conf, --channel, --maintenance-window flags.
Precedence: CLI > config file > package defaults.

update/cmd/apply.go gains four gates in order:
1. Maintenance window — checked locally before any HTTP work; skipped
   with --force.
2. Channel — refused if metadata.channel doesn't match opts.Channel.
3. Architecture — refused if metadata.architecture != runtime.GOARCH.
4. Min compatible version — refused if FromVersion < min_compatible.
All gate failures transition state to Failed with a clear LastError.

cloud-init gains a top-level updates: block (Server, Channel,
MaintenanceWindow, PubKey). cloud-init.ApplyUpdates writes
/etc/kubesolo/update.conf from those fields on first boot. Empty block
leaves any existing file alone (so hand-edited update.conf survives a
reboot without cloud-init re-applying). 4 new tests cover empty / all /
partial / parent-dir-creation cases. full-config.yaml example updated.

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

cloud-init/updates_test.go

@@ -0,0 +1,81 @@
+package cloudinit
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestApplyUpdatesEmptyConfigSkipsWrite(t *testing.T) {
+	confPath := filepath.Join(t.TempDir(), "update.conf")
+	cfg := &Config{} // Updates block default-zero
+	if err := ApplyUpdates(cfg, confPath); err != nil {
+		t.Fatalf("apply: %v", err)
+	}
+	if _, err := os.Stat(confPath); !os.IsNotExist(err) {
+		t.Errorf("expected no file when cloud-init Updates is empty, got %v", err)
+	}
+}
+
+func TestApplyUpdatesAllFields(t *testing.T) {
+	confPath := filepath.Join(t.TempDir(), "update.conf")
+	cfg := &Config{Updates: UpdatesConfig{
+		Server:            "https://updates.example.com",
+		Channel:           "stable",
+		MaintenanceWindow: "03:00-05:00",
+		PubKey:            "/etc/kubesolo/pub.hex",
+	}}
+	if err := ApplyUpdates(cfg, confPath); err != nil {
+		t.Fatalf("apply: %v", err)
+	}
+	data, err := os.ReadFile(confPath)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	out := string(data)
+
+	wants := []string{
+		"server = https://updates.example.com",
+		"channel = stable",
+		"maintenance_window = 03:00-05:00",
+		"pubkey = /etc/kubesolo/pub.hex",
+	}
+	for _, w := range wants {
+		if !strings.Contains(out, w) {
+			t.Errorf("update.conf missing %q in output:\n%s", w, out)
+		}
+	}
+}
+
+func TestApplyUpdatesPartialFields(t *testing.T) {
+	// Only server set — others should be omitted from the file, not written
+	// as blank values.
+	confPath := filepath.Join(t.TempDir(), "update.conf")
+	cfg := &Config{Updates: UpdatesConfig{Server: "https://x.example.com"}}
+	if err := ApplyUpdates(cfg, confPath); err != nil {
+		t.Fatalf("apply: %v", err)
+	}
+	data, _ := os.ReadFile(confPath)
+	out := string(data)
+	if !strings.Contains(out, "server = https://x.example.com") {
+		t.Errorf("missing server line:\n%s", out)
+	}
+	for _, unwanted := range []string{"channel = ", "maintenance_window = ", "pubkey = "} {
+		if strings.Contains(out, unwanted) {
+			t.Errorf("unexpected empty line %q present in:\n%s", unwanted, out)
+		}
+	}
+}
+
+func TestApplyUpdatesCreatesParentDir(t *testing.T) {
+	// /etc/kubesolo may not exist on first boot before cloud-init runs.
+	confPath := filepath.Join(t.TempDir(), "nested", "kubesolo", "update.conf")
+	cfg := &Config{Updates: UpdatesConfig{Server: "https://x"}}
+	if err := ApplyUpdates(cfg, confPath); err != nil {
+		t.Fatalf("apply: %v", err)
+	}
+	if _, err := os.Stat(confPath); err != nil {
+		t.Errorf("file not created: %v", err)
+	}
+}