feat(notification): list/read/count + preferences

[omattsson]

Summary

• GET /api/v1/notifications → stackctl notification list
• GET /api/v1/notifications/count → stackctl notification count
• POST /api/v1/notifications/:id/read → stackctl notification read <id>
• POST /api/v1/notifications/read-all → stackctl notification read-all
• GET /api/v1/notifications/preferences → stackctl notification prefs get
• PUT /api/v1/notifications/preferences → stackctl notification prefs set --from-file prefs.json

Files to touch

• New: cli/cmd/notification.go, cli/cmd/notification_test.go.
• cli/pkg/types/types.go — Notification, NotificationPreferences.
• cli/pkg/client/client.go — six methods.

Tests (all three layers required)

• Unit: cli/cmd/notification_test.go + cli/pkg/client/client_test.go — all six verbs, all output modes.
• Integration: new cli/test/integration/notification_integration_test.go — Cobra in-process count → read-all → count round-trip; prefs get | prefs set round-trip.
• E2E: cli/test/e2e/cli_e2e_test.go — stackctl notification list -o json happy path.

Acceptance

• Unread count matches server after read/read-all.
• prefs get | jq … | prefs set round-trip works.

Tracks #59