docs: the hive dashboard and agent page shrink to overviews

The hive dashboard and the per-agent page lose their control-by-control UI
description and endpoint tables from the handbook. The full text moves to
4-implementation/web-ui/dashboard.md and agent-page.md, and the handbook pages
keep what an operator needs to find and use them. Code comments that named the
old sections point at the new pages.

docs/2-handbook/hives/README.md

@@ -99,15 +99,16 @@ For the code, these jump straight to the relevant section:
 - **"How does the dashboard SPA stay live without polling?"** →
   [`shape.md`](../../4-implementation/web-ui/shape.md) (SSE multiplexing,
   Worker-death self-heal, atomic repaint).
-- **"What does a container row contain?"** → [`dashboard.md`](dashboard.md)
-  (Container row, Selection bar).
+- **"What does a container row contain?"** →
+  [`dashboard.md`](../../4-implementation/web-ui/dashboard.md) (Container row,
+  Selection bar).
 - **"What endpoints does the dashboard expose?"** →
-  [`dashboard.md`](dashboard.md) (Dashboard endpoints, Dashboard event
-  channel).
+  [`dashboard.md`](../../4-implementation/web-ui/dashboard.md) (Dashboard endpoints,
+  Dashboard event channel).
 - **"How does the per-agent terminal render tool calls?"** →
   [`terminal-rendering.md`](../../4-implementation/web-ui/terminal-rendering.md)
   (the full row taxonomy and dispatch walkthrough); for a summary see
-  [`web-page.md`](../agents/web-page.md) (Per-stream rendering).
+  [`agent-page.md`](../../4-implementation/web-ui/agent-page.md) (Per-stream rendering).
 - **"Which CSS variable to use, and where are the colours defined?"** →
   [`css-vars.md`](../../4-implementation/web-ui/css-vars.md) (Palette, the
   single-source `base.css` rule).