agent.css: delete dead .overflow-btn/.overflow-menu/.overflow-item rules

`OverflowMenu.tsx`/`.css` were deleted (rebuild button removed outright,
dashboard link moved into `MetaNav`), but `agent.css` still carried the
whole trigger+popover+item rule set — grepped the entire `agent`
package, none of these classnames appear in any current markup.

Also fixed three tombstone comments elsewhere in the file that pointed
at the now-also-deleted `.overflow-item-*` classes as "where it moved
to" — they'd have dangled once those rules were gone too. And two stale
doc claims caught in the same sweep: `docs/web-ui/agent.md` still listed
`OverflowMenu.tsx` in the header's component tree and described "the
overflow menu" as a live effort-picker location.

docs/web-ui/agent.md

@@ -11,8 +11,8 @@ panel for flyouts and long content.
 ## Header
 
 Preact component tree (`Header.tsx` + `StatusChips.tsx` +
-`MetaNav.tsx` + `OverflowMenu.tsx` + `HeaderPill.tsx`, wired together
-in `Root.tsx`) — see `frontend/packages/agent/src/components/`. This
+`MetaNav.tsx` + `HeaderPill.tsx`, wired together in `Root.tsx`) — see
+`frontend/packages/agent/src/components/`. This
 section describes the rendered result, not the DOM ids the pre-Preact
 page used (there are none any more — every element is component
 output, not something a selector reaches by id).
@@ -253,8 +253,8 @@ Slash commands today:
   restart / rebuild.
 - `/effort <level>` — `POST /api/effort` setting the claude effort
   level (`low` / `medium` / `high` / `xhigh` / `max`). Takes effect on the next
-  turn. The overflow menu surfaces an effort picker that calls the
-  same endpoint; both stay in sync via `StateSnapshot.effort`.
+  turn. `StatusChips`' effort badge calls the same endpoint; both stay
+  in sync via `StateSnapshot.effort`.
 - `/new-session` — `POST /api/new-session` (confirms first).
   Arms a one-shot on the Bus; next turn runs without
   `--continue`, dropping the resume session entirely.