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.

docs/web-ui/agent.md

@@ -69,29 +69,34 @@ structurally rather than for one specific trigger. Two columns:
     unprivileged process can't make directly).
   - **Inbox / todos pills** (`📬 inbox · N` / `📋 todos · N`): hidden
     when empty; click opens the matching flyout in the side panel.
-  - **Links badge** (`🔗`): opens a popover listing this agent's
-    `agent_links()`-sourced navigation (stats, screen when VNC is
-    enabled, forge profile + config-repo mirror when the agent has a
-    forge account, any `hyperhive.dashboardLinks` extras) as real
-    `<a>` elements — not a `Dropdown`-style command list, so
-    ctrl/middle-click and "copy link address" keep working. Each
-    `AgentLink.kind` resolves differently: `container` → same-origin
-    path; `forge` → `state.forge_public_url + url`, and the link is
-    dropped entirely when that's unset (never guessed from
+  - **Links badge** (`🔗`): opens a popover listing `↑ dashboard`
+    (back-link to the host dashboard, `${dashboardBase}dashboard.html`)
+    followed by this agent's `agent_links()`-sourced navigation (stats,
+    screen when VNC is enabled, forge profile + config-repo mirror when
+    the agent has a forge account, any `hyperhive.dashboardLinks`
+    extras) — all as real `<a>` elements, not a `Dropdown`-style
+    command list, so ctrl/middle-click and "copy link address" keep
+    working. Each `AgentLink.kind` resolves differently: `container` →
+    same-origin path; `forge` → `state.forge_public_url + url`, and the
+    link is dropped entirely when that's unset (never guessed from
     `<host>:3000`); `external` → already absolute. Same source
     (`GET /api/state`'s `links` field) also feeds
     `DashboardState.links` for the dashboard card's icon strip —
     `agent_links()` in hive-agent is the single source of truth for
-    both.
-  - **Overflow badge** (`⋯`): a `Dropdown` with two rows —
-    `↑ dashboard` (link) and `↻ rebuild container` (select-twice
-    confirm, same action as the dashboard R3BU1LD button). Everything
-    else that used to live here (model/effort pickers, new-session,
-    logout) has a better home now: pickers are real badges above, and
-    `/new-session` / `/logout` are typed slash commands with their own
-    type-twice confirm (see below) — a modal doesn't fit a text-input
-    flow, and burying rare-but-important actions in one flat menu was
-    the design guide's own named anti-example.
+    both. There is no separate overflow (`⋯`) menu any more — it used
+    to hold exactly this dashboard link plus a rebuild-container action
+    (mara, hyperhive#3704: "remove rebuild button, move link to
+    dashboards into links menu") — rebuild had no real discoverability
+    need of its own (the dashboard's own R3BU1LD button already covers
+    it) so it's gone outright, and the dashboard link moved here,
+    leaving nothing to justify a separate menu. Everything else that
+    used to live in the old overflow menu (model/effort pickers,
+    new-session, logout) already had a better home before this: pickers
+    are real badges above, and `/new-session` / `/logout` are typed
+    slash commands with their own type-twice confirm (see below) — a
+    modal doesn't fit a text-input flow, and burying rare-but-important
+    actions in one flat menu was the design guide's own named
+    anti-example.
   - No header cancel-turn button any more — `/cancel` (slash command,
     below) is the only path; the turn-loop state badge already shows
     `thinking` as the discoverability cue.