Pairs with damocles PR #566 (broker primitive + dashboard
`POST /api/agent/{name}/mark-all-read` route). The agent's
per-container inbox side-panel now gets a header row with a
`✓ mark all read` button that:
- confirms via a one-line dialog (the action is destructive: any
pending broker message for this agent is acked, the harness
won't receive a wake-prompt for them)
- POSTs to the host dashboard (cross-origin, same pattern as the
existing operator-answer flow on this page)
- surfaces `{ marked: N }` in an inline status pill, then triggers
a `refreshState` so any state-derived surfaces re-read fresh
- stays out of the way when the inbox is empty (only renders above
a non-empty rows list)
Note: `recent_for` returns the most-recent-N messages regardless of
ack state, so clicking does NOT visually empty the rows list. The
status pill ("✓ marked N as read") is the operator-facing
confirmation; the next `turn_start` will show `0 unread` in its
badge. Tooltip on the button calls this out so the operator isn't
surprised the row list stays put.
CSS mirrors the existing answer-form button family (mauve hover on
bg-elev background) so it reads as a peer affordance, with a
border-bottom separating it from the message list.
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