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).

docs/gotchas.md

@@ -256,7 +256,7 @@ derivation's `nativeBuildInputs`.
 `nix/templates/weston-vnc.nix` adds an optional Weston Wayland
 compositor with the VNC backend, surfaced as
 `hyperhive.gui.enable = true` per-agent. The harness's
-`/screen/ws` WebSocket relay (`docs/web-ui.md::Per-agent endpoints`)
+`/screen/ws` WebSocket relay (`docs/web-ui/agent.md::Per-agent endpoints`)
 connects to the compositor at `127.0.0.1:<vnc_port>`.
 
 - **Port allocation**: deterministic FNV-1a of the agent name