docs(#727): split docs/web-ui.md into shape / dashboard / agent

docs/web-ui.md (1315 lines) split into three sub-files:
- docs/web-ui/shape.md   — shared SPA skeleton, SSE multiplexing,
  Worker-death self-heal, terminal pane, listener bind, relative
  paths, atomic repaint, side panel
- docs/web-ui/dashboard.md — SW4RM/Y3R/SYST3M/SCH3DUL3S/S3TT1NGS
  tabs, container row, topology tree, selection bar, approval card,
  dashboard endpoints + event channel
- docs/web-ui/agent.md   — header, terminal, composer, inbox, live
  view, slash commands, per-agent endpoints, stats page

docs/web-ui.md replaced with a thin index linking all three.

Section anchors in docs (gateway.md, gotchas.md), Rust doc comments
(hive-ag3nt/src/web_ui.rs), and nix/templates/weston-vnc.nix updated
to point at the correct sub-file. README and CLAUDE.md file-map
updated with sub-file links. Inline // comments in frontend source
left unchanged (they reference the index which redirects to the right
sub-file).

CLAUDE.md

@@ -298,7 +298,13 @@ nix/
 docs/
   conventions.md       naming, identity=socket, async forms, commit style
   gotchas.md           NixOS / nspawn quirks and lessons learned
-  web-ui.md            dashboard + per-agent page layouts and endpoints
+  web-ui.md            index → web-ui/shape.md (shared skeleton, SSE,
+                       terminal, listener bind, relative paths, atomic
+                       repaint, side panel), web-ui/dashboard.md (tab
+                       contents, container row, topology, selection bar,
+                       approval card, endpoints + event channel),
+                       web-ui/agent.md (header, terminal, composer,
+                       inbox, live view, per-agent endpoints, stats)
   turn-loop.md         claude invocation, wake prompt, MCP tool surface
   approvals.md         approval flow, manager policy, helper events
   persistence.md       sqlite dbs, retention, state dir layout
@@ -324,7 +330,10 @@ Pick the doc that matches your task. None depend on the others —
 read them à la carte.
 
 - **"What does the dashboard look like?"** →
-  [`docs/web-ui.md`](docs/web-ui.md).
+  [`docs/web-ui.md`](docs/web-ui.md) (index; sub-pages:
+  [`shape`](docs/web-ui/shape.md),
+  [`dashboard`](docs/web-ui/dashboard.md),
+  [`agent`](docs/web-ui/agent.md)).
 - **"How does the per-agent terminal classify + colour
   events?"** → [`docs/terminal-rendering.md`](docs/terminal-rendering.md).
 - **"How does claude get its prompt and what tools does it have?"** →