docs: retire the stray top-level web-ui.md, fold it into web-ui/README.md

docs/web-ui.md duplicated the web-ui/ directory name at the top level --
the only such collision in docs/ (every other subsystem has just a
directory, no sibling <dir>.md file). That's exactly why it rendered
outside the directory structure in the docs site nav (mara's report,
hyperhive#4054): the site build walks docs/ generically with no
special-casing, so a loose top-level file next to a same-named
directory shows up as its own flat top-level entry instead of nesting
under that directory's section.

web-ui.md's own first paragraph already said as much -- 'This doc has
been split for readability... start at web-ui/README.md instead.' It
was a leftover pointer from before the split, not a page carrying
unique content on its own merit.

Folded its two sections web-ui/README.md didn't already have (the
swarm-ui design-guide link, and the task-oriented 'reading paths'
quick-lookup list) into web-ui/README.md's existing 'More depth'
section, then deleted the stray file and repointed every real
reference at it: 3 in-tree doc cross-links, 3 doc prose mentions
(retargeted to the more specific dashboard.md/shape.md sub-page each
one was actually about), and ~28 frontend source comments
(dashboard/agent/shared packages) that cited it as
'docs/web-ui.md::<heading>' for implementation context -- retargeted
each to whichever of dashboard.md/shape.md/agent.md actually carries
that heading now, verified against each file's real heading list
rather than guessed.

Verified via scripts/check-doc-refs.sh (the same lint CI runs): 0 dead
pointers, both before write (confirming the tree was clean beforehand)
and after (confirming nothing broke).

docs/agent-lifecycle/approvals.md

@@ -448,7 +448,7 @@ Two visible consequences:
   on every `rebuild_queue_changed` tick), not on the approvals panel
   (which already moved the row to "approved"). A long meta-update
   cascade renders as a parent DAG with one child rebuild per affected
-  agent — see `docs/web-ui.md` for the layout.
+  agent — see `docs/web-ui/dashboard.md` for the layout.
 - **Cancellation**: the dashboard's *× cancel* button on a still-queued
   DAG calls `POST /api/rebuild-queue/{id}/cancel`, which flips it to
   `Cancelled` before any node runs (and fails the approval row instead
@@ -485,7 +485,7 @@ agent.
 
 The dashboard deep-links into this org — a `config repo` link
 per container row and a `review PR on forge` link per config-PR
-approval card. See `docs/web-ui.md`.
+approval card. See `docs/web-ui/dashboard.md`.
 
 ### Submitting agent's view of config repos