v1.29.0

Added

• Opt-in dashboard Upgrade button. When dashboard.control is enabled and a
  newer release is available, the dashboard shows a one-click Upgrade now
  button. It preserves the non-root dashboard's security model: the dashboard
  has no host or Docker access and only writes a request marker to its state
  volume. A new host-side agent, ./stack upgrade-agent, watches for the marker
  and runs ./stack upgrade with the right privileges (run it under systemd —
  see docs/operations.md). Off by
  default; without the agent running, the button does nothing. This is the
  dashboard button the 1.28.0 notes deferred — now possible without giving
  the dashboard host/docker control. Covered by unit tests (endpoint gating,
  CSRF) and an end-to-end CI test (the agent consumes a button-written request
  and upgrades).