fix: ignore drifting timestamp in stories (#25429)

The `SectionHeadersCollapse` story (and every other
Chromatic-snapshotted story rendering the agents sidebar) was flaky
because each chat row renders `shortRelativeTime(chat.updated_at)` —
`"now"`, `"5m"`, `"7h"`, `"1d"`, etc. The values are computed against
the live wall clock from fixed fixture timestamps, so the rendered text
drifts on every run (e.g. `"7h"` → `"now"` mid-day) and Chromatic diffs
the change.

This wraps the timestamp in `<span data-chromatic="ignore">`, the
dominant codebase convention for time-varying text.
`data-chromatic="ignore"` appears 35 times across 20 files in
`site/src`, including `utils/schedule.tsx:139,153`,
`components/LastSeen/LastSeen.tsx:42`,
`modules/provisioners/Provisioner.tsx:80`,
`pages/OrganizationSettingsPage/.../ProvisionerRow.tsx:119`,
`JobRow.tsx:139`, `ProvisionerKeyRow.tsx:81`,
`pages/AIBridgePage/RequestLogsPage/RequestLogsRow/RequestLogsRow.tsx`
(multiple rows), `pages/HealthPage/WorkspaceProxyPage.tsx:144`, and
`pages/UserSettingsPage/TokensPage/TokensPageView.tsx:117`. By
comparison, only one component (`AppStatuses`) plumbs a reference date
through props, and there is no `Date.now`-stubbing precedent that would
actually stabilize `dayjs()`-based output like `shortRelativeTime`.

The wrap is scoped tightly: only the text node returned by
`shortRelativeTime` is ignored, so the sibling unread-indicator dot and
surrounding layout still participate in snapshots.

site/src/pages/AgentsPage/components/Sidebar/AgentsSidebar.tsx

@@ -769,7 +769,11 @@ const ChatTreeNode: FC<ChatTreeNodeProps> = ({ chat, isChildNode }) => {
 												aria-hidden="true"
 											/>
 										) : (
-											shortRelativeTime(chat.updated_at)
+											// data-chromatic="ignore" keeps Chromatic snapshots
+											// stable as wall-clock time advances (e.g. "now" → "5m").
+											<span data-chromatic="ignore">
+												{shortRelativeTime(chat.updated_at)}
+											</span>
 										)}
 									</span>
 									<DropdownMenu>