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).

README.md

@@ -37,7 +37,7 @@ Depth lives in [`docs/`](docs/) — pick the one matching your task:
 
 | reading path | doc |
 | --- | --- |
-| dashboard layout + endpoints | [`docs/web-ui.md`](docs/web-ui.md) |
+| dashboard layout + endpoints | [`docs/web-ui.md`](docs/web-ui.md) ([shape](docs/web-ui/shape.md) · [dashboard](docs/web-ui/dashboard.md) · [agent](docs/web-ui/agent.md)) |
 | claude turn loop + MCP tools | [`docs/turn-loop.md`](docs/turn-loop.md) |
 | config-edit + approval state machine | [`docs/approvals.md`](docs/approvals.md) |
 | what survives destroy / purge / restart | [`docs/persistence.md`](docs/persistence.md) |