agent: remove rebuild button, move dashboard link into the links menu

mara (#3704): 'remove rebuild button, move link to dashboards into
links menu.'

The overflow (⋯) menu existed for exactly two items: the dashboard
back-link and a rebuild-container action. Rebuild is gone outright —
the dashboard's own R3BU1LD button already covers it, this was just a
rarely-used shortcut not worth its own menu. The dashboard link moves
into MetaNav's links popover (now the first item, above stats/forge/
config/extras) instead. With both gone, OverflowMenu had nothing left
to justify existing as a separate component — deleted along with its
CSS and the now-unused rebuildAction.ts (only consumer).

MetaNav gained a dashboardBase prop (Root.tsx already computes this
via resolveDashboardBase for InboxPanel/pause — reused, not
duplicated) and renders the dashboard link as a real <a>, same
treatment as every other item in that popover — no dangling
window.open()-only affordance.

Updated docs/web-ui/agent.md's Header section and the couple of
now-stale OverflowMenu references in index.html's/MetaNav.css's own
comments.

Verified: header now shows a single trailing icon-badge (was two),
popover opens with dashboard first then the agent_links() set.
tsc --noEmit clean, build clean, both pre-push lints clean.

frontend/packages/agent/src/components/MetaNav.css

@@ -3,10 +3,8 @@
    same "reuse the values, not the component" call `.login-card`
    (LoginFlow.css) already made, for the same reason: this popover's
    items are real `<a>` tags, not `Dropdown`'s command-dispatch
-   `<button>`s. Right-anchored, same as OverflowMenu.css's
-   `.header-overflow-anchor` — MetaNav sits in the same right-hand
-   pills cluster, so a left anchor would push it off-screen the same
-   way. */
+   `<button>`s. Right-anchored — MetaNav sits in the header's right-hand
+   pills cluster, so a left anchor would push it off-screen. */
 .meta-nav-anchor {
   position: relative;
   display: inline-block;