docs(web-ui): document turn start/end times + duration on the agent terminal

Follow-up to the turn-time feature: the terminal now stamps a wall-clock
time on the turn-start / turn-end rows and an elapsed duration on
turn-end, sourced from a per-event `ts` (unix seconds) that the backend
serializes as a flattened sibling of the event tag on both the live SSE
frame and each history row.

- terminal-rendering.md: add the `.turn-time` row to the taxonomy table
  + a note on the ts source and the graceful degradation when a frame
  omits ts.
- web-ui/agent.md: note the ts sibling on the emitted events, and the
  time/duration suffixes on the TurnStart / TurnEnd renderings.

docs/web-ui/agent.md

@@ -182,7 +182,9 @@ Each agent runs an `events::Bus`: a `tokio::sync::broadcast<LiveEvent>`
 plus a sqlite-backed history at `/state/hyperhive-events.sqlite`.
 The harness emits `TurnStart { from, body, unread }`,
 `Stream(value)` (one per parsed stream-json line), `Note`,
-`TurnEnd { ok, note }`. The web UI:
+`TurnEnd { ok, note }`. Each event also carries a `ts` (unix
+seconds) — a flattened sibling of the event tag on both the live
+SSE frame and the replayed history rows. The web UI:
 
 - fetches `GET /events/history` on page load and replays the last
   2000 events (oldest first, with `.no-anim` so they don't
@@ -212,8 +214,12 @@ Per-stream rendering:
 - `Stream` `system init`, `result`, `rate_limit_event` are
   dropped — too noisy.
 - `Note` → `· text`.
-- `TurnEnd` → `✓ turn ok` / `✗ turn fail — note`, triggers a
-  `refreshState()`.
+- `TurnStart` → `◆ TURN ← <from>` with the wake-prompt body; a
+  muted `· HH:MM:SS` time suffix from the event `ts`.
+- `TurnEnd` → `✓ turn ok` / `✗ turn fail — note`, with a
+  `· HH:MM:SS · <duration>` suffix (duration = end − the paired
+  turn-start), triggers a `refreshState()`. The time suffix is
+  guarded on a numeric `ts`, so rows degrade cleanly when absent.
 
 ## Terminal-embedded prompt