docs(web-ui): add an operator-facing README as the subdir landing page

docs/web-ui/ had four pages, all written as deep implementation
reference (dashboard.md alone is 1466 lines of wire shapes and DOM
mechanics) - there was nothing here written for an operator actually
using the dashboard day to day.

Add docs/web-ui/README.md: genuinely task-oriented content (what the
dashboard is, where the tabs are, the things you'd actually do -
checking an agent, answering a pending question, approving a config
change, granting a permission, reading logs) with pointers into the
existing pages for implementation depth. Point docs/web-ui.md at it as
the operator starting point.

Deliberately not touching dashboard.md/agent.md/shape.md/css-vars.md -
reflowing 1466 lines of dense, working reference content in place
turned out to be a much bigger and more error-prone task than "lead
with the user fact" suggested at a glance (see hyperhive#1898). The
subdir's landing page carries the user-facing content; the existing
pages stay exactly as they are, as the depth layer underneath it.

Verified the new page actually renders as the subdir's index via the
website repo's new subdir-landing-page support (hyperhive/website
PR #46): a real build of the prose-docs derivation confirms
web-ui/index.html now exists, the root index links straight to it, and
every internal link resolves correctly.

docs/web-ui.md

@@ -6,7 +6,10 @@ and the per-agent UIs (every container hashes into :8100-8999 via
 Both are SPAs — `GET /` returns a static shell, `/api/state`
 returns JSON, JS renders. No full-page reloads.
 
-This doc has been split for readability. Pick the section you need:
+This doc has been split for readability. Pick the section you need —
+or, if you're the operator looking for how to actually *use* the
+dashboard rather than how it's built, start at
+**[web-ui/README.md](web-ui/README.md)** instead.
 
 - **[Shape (shared by both)](web-ui/shape.md)** — shared SPA
   skeleton, SSE multiplexing, terminal pane, listener bind,