Remove or restore the unreachable docs sidebar tree section

[alexeygrigorev]

Raw finding from #192 implementation and verification (orchestrator intake).

frontend/src/styles.css:497-499 sets .docs-sidebar-only { display: none; } unconditionally, with no override anywhere in the codebase. That makes the entire <section class="tree-section docs-sidebar-only"> at frontend/index.html:102 unreachable UI — the "Processes" label, #tree-skeleton, and #doc-tree.

Meanwhile renderTree() still renders into #doc-tree on every refresh, so the app is doing layout work for markup no operator can see.

How it was found

While verifying #192, the Tester disproved a spec assertion that waited on #tree-skeleton being hidden. Forcing the element back into its loading shape (hidden = false, exactly what loadDocuments() sets before awaiting GET /docs) still reported hidden with display: none, boxWidth: 0, boxHeight: 0, and the wait returned in 2ms. The Software Engineer independently reproduced this.

Why it matters

This is the same defect class as the #status-text / .library-heading finding that motivated #192 itself: markup the code keeps updating, that CSS makes permanently invisible. Dead UI of this kind silently invalidates tests that reference it — it already produced one tautological assertion that reached Tester review.

To settle during grooming

• Is the sidebar tree intended to return, or is it superseded by the #/processes surface?
• If superseded: delete the section, its CSS, and the renderTree() path that feeds it.
• If intended: restore visibility and give it real coverage.
• Audit for other display: none markup the app still renders into, since this is now the second instance found.

Related: #192 (docs outage states), #193 (renderHonestState inline copy).