docs: migrate tabs.js topology-tree prose to docs/web-ui.md (#712 pattern check)

First slice of #712 (dashboard SPA frontend → docs/). Shrinks the
buildAgentTree + treePrefixDom comment blocks in tabs.js to one-line
docs/ references; adds a new 'Topology tree' subsection to
docs/web-ui.md under '### Container row' carrying the substantive
prose.

Net effect on this region:
- 6 #NNN tracking cookies dropped (#363, #361, #388)
- 30 lines of substantive design comments collapsed to 6 lines of
  references in tabs.js
- 35 new lines in docs/web-ui.md (the migrated prose, restructured
  into a single coherent subsection rather than two adjacent comment
  blocks)
- functional code unchanged; build clean

Deliberately small pattern-check (one cohesive subsystem) before
expanding to the rest of #712 (~140 more refs across tabs.js +
dashboard.css + index.html + common.js + flow.js). Reviewers can
sanity-check the shape (where prose lands, how short the in-code
ref becomes, whether section nesting reads well) before I scale.

refs #712

docs/web-ui.md

@@ -425,6 +425,41 @@ body sits to the right with three stacked lines
 agent is stale. Banner pulses on each broker SSE event
 (`pulseBanner` with a 4s grace timer).
 
+#### Topology tree
+
+Container rows render as a forest, not a flat list — each agent
+sits indented under its declared parent. `tabs.js::buildAgentTree`
+walks `ContainerView.parent` for every container in the snapshot
+and produces a render order with per-row depth + sibling-position
+info:
+
+- Top-level rows are agents with `parent = null` OR a parent that
+  doesn't appear in the container map (orphans get hoisted to root
+  so they're still visible).
+- Within each level children sort alphabetically by name; roots
+  likewise.
+- Cycle safety: any container not reached during the root-walk is
+  appended at the end as a root, so no agent ever silently
+  disappears from the list when the topology JSON is malformed.
+- The pre-topology rendering shape (every container at depth 0,
+  flat list) collapses to the same visual today when no parent
+  field is set — bit-identical fallback path.
+
+The per-row prefix column (`.tree-prefix`) is **DOM-painted, not
+text-glyph-painted**. Each indent lane is its own positioned
+`<span>` so CSS can draw full-height vertical bars that bridge the
+gap between sibling rows; using text box-drawing characters
+(`├─`, `└─`, `│  `) only paints one text-line tall and leaves
+visible breaks between the taller-than-one-line container cards.
+The bars come in two flavours: continuation (the ancestor's
+subtree extends below this row → vertical line top→bottom) or
+blank (ancestor was the last sibling at its level → no line
+needed). The joint at the row's own depth column is `├` (more
+siblings below) or `└` (last sibling at this depth — vertical
+stops at the row's icon midline).
+
+
+
 ### Selection bar
 
 When one or more agents are selected (via icon click), a sticky

frontend/packages/dashboard/src/tabs.js

@@ -370,20 +370,9 @@ window.marked = marked;
   }
 
   // ─── state rendering ────────────────────────────────────────────────────
-  // ─── agent topology (#363) ──────────────────────────────────────────────
-  // Build a forest from `ContainerView.parent` and walk depth-first to
-  // produce a render order with per-row depth + sibling-position info.
-  // Top-level (parent = null OR parent not in the container map) are
-  // roots. Within each level, children are sorted alphabetically by
-  // name; roots likewise. Cycles in the parent graph (malformed config)
-  // are tolerated: any container not reached via root-walk is appended
-  // as a root at the end, so no agent ever disappears from the list.
-  //
-  // For the visual: each row gets a textual prefix column (`├─`, `└─`,
-  // continuation `│  ` or padding `   ` for ancestor columns). When
-  // every container has `parent = null` (today's pre-#361 state), the
-  // tree collapses to a flat list with no glyphs and no indent — bit-
-  // identical to the legacy render.
+  // ─── agent topology ─────────────────────────────────────────────────
+  // See docs/web-ui.md::Topology tree for the rendering contract
+  // (forest walk, alphabetical sort, orphan + cycle handling).
   function buildAgentTree(containers) {
     const byName = new Map();
     for (const c of containers) byName.set(c.name, c);
@@ -421,16 +410,9 @@ window.marked = marked;
     }
     return out;
   }
-  // Builds the .tree-prefix DOM for a row at the given depth. Each
-  // lane is its own positioned child so CSS can paint full-height
-  // vertical bars that bridge the gap between sibling rows — text
-  // box-drawing glyphs only paint one text-line tall, which left
-  // visible breaks between rows once we grew taller-than-one-line
-  // container cards (#388). Ancestor lanes are either continuation
-  // (vertical bar top→bottom+gap) or blank; the joint at this row's
-  // own depth is ├ (branch — vertical continues below) or └ (last —
-  // vertical stops at the row's icon midline). CSS at
-  // `.container-row .tree-prefix` paints the bars + horizontal stub.
+  // 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
+  // (one positioned <span> per lane) rather than text-glyph-painted.
   function treePrefixDom({ depth, ancestorIsLast, isLast }) {
     if (depth === 0) return null;
     const prefix = el('span', { class: 'tree-prefix', 'aria-hidden': 'true' });