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).

nix/templates/weston-vnc.nix

@@ -13,7 +13,7 @@
   # Port allocation, weston bind-address quirk, PAM service name, the
   # Type=simple choice, idle-time=0: all in
   # docs/gotchas.md::Weston VNC compositor.
-  # Harness-side WebSocket relay shape: docs/web-ui.md::Per-agent
+  # Harness-side WebSocket relay shape: docs/web-ui/agent.md::Per-agent
   # endpoints (`/screen` + `/screen/ws`).
 
   options.hyperhive.gui.enable = lib.mkOption {