Reserve the Chief of Staff two-pane shell with a PageSkeleton split layout

[atomantic]

Summary

/cos is an isFullWidth route, so Layout hands it a bare relative overflow-hidden <main>. Its loading guard rendered a centered BrailleSpinner inside an h-64 box, which reserved none of the loaded shell — the whole two-pane page (320px agent rail + tab/content pane) jumped into place on first paint.

PageSkeleton had no two-pane mode (layout only modelled stack/grid), so this takes option 1 from the issue and adds one rather than hand-rolling a page-local skeleton:

• PageSkeleton gains layout="split" — a two-pane shell that owns the whole page: a fixed-width side rail beside a flexible main pane that owns the scroll under fullHeight. Because it owns the shell it ignores header (a two-pane page's title lives inside a pane) and renders the tab strip inside the main pane, where a two-pane page actually puts it. Knobs mirror the loaded layout: sideCollapsed (desktop rail collapsed to a zero-width track while the mobile band stays), splitColsClass (grid tracks, defaulted off sideCollapsed so a collapsed rail can't reserve 320px of nothing), sideClassName, sideHero (avatar/portrait circle), sideBlocks + sideBlockColsClass (nav list at 1 col, stat grid at 2). Below lg the rail stacks above the main pane, same as the loaded page, so the mobile band is reserved too.
• Chief of Staff uses it. desktopPanelCollapsed is read from localStorage before the first fetch resolves, so the skeleton already holds the right rail width (lg:grid-cols-[320px_1fr] or lg:grid-cols-[0px_1fr]) and reserves the 2×2 stat grid, the avatar, and the TABS.length tab strip.
• Small DRY pass: the stacked-card block is extracted once (stackedCards) and shared by the stack and split bodies, and the stack/grid body tree is now built after the split early return so a two-pane page doesn't allocate a tree it never renders.

Scoped to Chief of Staff and the shared primitive it needed — the sub-region spinners in #4147 are deliberately left alone.

Test plan

• client/src/components/ui/PageSkeleton.test.jsx — new layout="split" block: reserves the two-pane tracks and drops the stack sidebar; collapses the rail track by default under sideCollapsed; takes a caller override over that derived default; renders the tab strip inside the main pane (asserted by locating the strip's pane and finding the cards in it); reserves hero + rail blocks in the requested column count; omits both when not requested; sideCollapsed keeps a zero-width desktop track plus the lg:hidden mobile band; h-full and the main pane's overflow-y-auto only under fullHeight; bodyClassName applies only when padded.
• client/src/pages/ChiefOfStaff.test.jsx — new guard holds the first getCosStatus fetch open and asserts the busy region is the two-pane grid (aria-busy, aria-label="Loading Chief of Staff", lg:grid-cols-[320px_1fr], h-full) and that the old h-64 spinner shell is gone. jsdom does no layout, so the guard pins the structure that reserves the dimensions.
• Full client suite: cd client && npx vitest run → 642 files / 7816 tests passing.
• npx biome lint --error-on-warnings clean on the four changed files.

Closes #4144