feat(milestones): add resilient loading and scoped recovery - #1139

Summary

Improve the milestones board's loading and failure behavior so users keep their context while the board resolves or recovers.

What changed

• Added MilestonesBoardSkeleton, shared by the App Router loading state and the page Suspense fallback.
• Matched the board's heading, filter toolbar, action controls, and milestone-card footprints to reduce layout shift.
• Added one polite role="status" loading announcement and hid decorative placeholders from assistive technology.
• Scoped MilestonesErrorBoundary instances around filters, actions, and the milestone list so one render failure does not remove healthy siblings.
• Added keyed retry remounting so Try again resets only the failed subtree.
• Added typed MILESTONES_SECTION_FAILED metadata with a stable section label and React component stack for observability.
• Removed raw thrown error messages from the fallback UI to prevent implementation details or sensitive values from leaking.
• Added unit and page-composition integration coverage for loading, isolation, retry recovery, accessibility, layout hooks, and reporting metadata.
• Added docs/MILESTONES_RESILIENCE.md with the design contract, accessibility guidance, testing strategy, and maintenance checklist.

Verification

• npm run lint on all changed source/test files: passed.
• Focused Jest suite: 4 suites, 43 tests passed.
• git diff --check: passed.
• Full repository npm run lint, npm test -- --runInBand, and npm run build are currently blocked by pre-existing unrelated issues on main, including merge-conflict markers and duplicate declarations in reputation/settings files, plus existing snapshot and dynamic-dialog failures. These are documented here so the baseline status is explicit.

Closes #1105