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).

frontend/packages/dashboard/src/swarm.js

@@ -44,7 +44,7 @@ const containerRowCache = new Map();
 const transientsState = new Map();
 
 // In-memory set of selected agent logical names backing the sticky
-// #selection-bar. See docs/web-ui.md::Selection bar for the
+// #selection-bar. See docs/web-ui/dashboard.md::Selection bar for the
 // interaction model (icon-click toggle, Esc/clear button drop,
 // tab-gated visibility).
 const selectionState = new Set();
@@ -227,7 +227,7 @@ function derivePortConflicts(containers) {
 }
 
 // ─── topology tree ───────────────────────────────────────────────────────────
-// See docs/web-ui.md::Topology tree for the rendering contract
+// See docs/web-ui/dashboard.md::Topology tree for the rendering contract
 // (forest walk, alphabetical sort, orphan + cycle handling).
 
 function buildAgentTree(containers) {
@@ -277,7 +277,7 @@ function buildAgentTree(containers) {
   return out;
 }
 // Builds the .tree-prefix DOM for a row at the given depth.
-// See docs/web-ui.md::Topology tree for why this is DOM-painted
+// See docs/web-ui/dashboard.md::Topology tree for why this is DOM-painted
 // (one positioned <span> per lane) rather than text-glyph-painted.
 function treePrefixDom({ depth, ancestorIsLast, isLast }) {
   if (depth === 0) return null;
@@ -650,7 +650,7 @@ function buildContainerLi(c, node, opts) {
 
   // Per-card action buttons (R3ST4RT / ST0P / ST4RT / R3BU1LD /
   // DESTR0Y / PURG3) live on the selection bar, not here — see
-  // docs/web-ui.md::Selection bar. The contextual `needs update ↻`
+  // docs/web-ui/dashboard.md::Selection bar. The contextual `needs update ↻`
   // chip in the head row stays — it's a state-hint, not an
   // action button.
 
@@ -830,7 +830,7 @@ export function renderContainers(s) {
 
 // ─── selection bar ───────────────────────────────────────────────────────────
 // Sticky-bottom strip; visible when >=1 agent selected on the SW4RM
-// tab. See docs/web-ui.md::Selection bar for the interaction model
+// tab. See docs/web-ui/dashboard.md::Selection bar for the interaction model
 // and the per-action availability rules (disabled-with-tooltip for
 // actions that don't apply to the full selection). Actions POST per
 // agent in a loop (endpoints are individually idempotent /