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/gateway.md

@@ -125,7 +125,7 @@ flip together: the primary agent-name link, the favicon fetch
 `/api/agent/<name>/links`. `forge`-kind nav-strip links still
 resolve against `http://<host>:3000` (separate sub-domain transition
 tracked by `forge.behindGateway`); `external`-kind links are
-already absolute. See `docs/web-ui.md::Container row` for the
+already absolute. See `docs/web-ui/dashboard.md::Container row` for the
 frontend-side derivation.
 
 ## Self-signed TLS (`selfSignedTls`)