agent page: dashboard back-link + last-turn timing chip

title bar grows a '↑ DASHB04RD' link next to the rebuild button —
opens the host dashboard in a new tab so the operator can pivot
between agents without losing the live tail. uses the dashboardPort
already plumbed via /api/state.

state row picks up a 'last turn 12.3s' chip that fills in when
state transitions away from thinking. format: ms / s.s / m s.
hidden until the first turn completes.

TODO.md

@@ -27,8 +27,12 @@ Pick anything from here when relevant. Cross-cutting design notes live in
 
 ## UI / UX
 
-- **Per-agent UI substance.** Show last N inbox messages, last turn timing,
-  link back to dashboard.
+- **Per-agent inbox view.** Show the last N messages addressed to
+  this agent on its page (the per-agent equivalent of the
+  dashboard's operator inbox). Needs a new wire request from agent
+  → host (host has the broker; agent doesn't); reuse the broker's
+  `recent_for` query. Last-turn timing + dashboard back-link
+  already shipped.
 - **State badge: compacting + napping states.** Idle/thinking already
   ship (driven from SSE turn_start/turn_end). Add `compacting 📦` and
   `napping 😴` once the `/compact` trigger and `nap` tool exist —