feat(fleet): set and show a keep deadline from the dashboard

[outofcoffee]

Set a keep (retention deadline) on a remote environment from the fleet dashboard, and show that deadline on the node's tile, detail screen, and metrics output.

Summary

• Adds a k key to the fleet dashboard (grid and detail view) that prompts for a duration and sets a keep on the selected remote environment, reporting the deadline the control plane set.
• Adds a Keeper capability interface to the fleet, implemented only by the remote node, and runs the keep through the existing per-node action machinery (tile spinner, one action per node, an immediate re-read on completion).
• The stats reply now carries retainUntil while the retention deadline is in the future, and a retain until line shows beside the last-active figure in the shared metrics render, so the dashboard tile, fleet metrics and remote metrics word it the same.
• Keep is remote-only: a local daemon has no idle sweep, so the key is not offered for it.

Implementation details

• The "is it still active" judgement sits in the stats Lambda once, not in the client: the control plane includes the deadline only while it is a time in the future, so a client or environment predating the field simply omits the line.
• The prompt is its own confirmation (no second one) and opens pre-filled with 4h. An entry that does not parse as a positive duration leaves the prompt open and shows the parse reason. A keep in flight is not abortable, unlike a cold cloud wake — a keep is one fast signed call, a wake has no deadline of its own.