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/index.html

@@ -9,7 +9,7 @@
   <link rel="stylesheet" href="static/theme.css">
   <link rel="stylesheet" href="static/agent.css">
   <!-- main.css (bundled component CSS: Badge/Dropdown/LoginFlow/MetaNav/
-       StatusChips/OverflowMenu/SidePanel/...) MUST load after agent.css —
+       StatusChips/SidePanel/...) MUST load after agent.css —
        `.login-card`/`.meta-nav-popover`/etc. share class-selector
        specificity with agent.css's legacy rules, so load order is what
        decides which one wins the cascade (see LoginFlow.css/MetaNav.css