good-first: announce keyboard Go to hint with aria-live

[unjica]

Summary

The floating “Go to… O overview · I issues …” hint should be announced when it appears.

File(s)

• apps/dashboard/app/components/dashboard/shell/DashboardKeyboardShortcuts.tsx

Tasks

• Add aria-live="polite" to the hint chip (already has role="status")
• Verify announcement when g is pressed

Acceptance criteria

• Screen readers announce the hint when shown
• No API/schema changes
• pnpm lint && pnpm test && pnpm build pass

Note

Related to but narrower than #211 (expand keyboard shortcuts).

Base branch

Open PRs against develop. See CONTRIBUTING.md.