docs: tune UI review guidance

[saagpatel]

What

• Tighten root Codex review guidance for support workflow UI states.
• Add nested frontend and UI-test review guidance.

Why

• Reduces generic UI comments and directs review toward keyboard/focus, responsive layout, privacy/status, and error-state risks.

How

• Added instruction-only AGENTS.md guidance under root, src, and tests/ui.

Testing

• Commands run: git diff --check
• Results: passed

Performance impact

• Bundle delta: none, instruction-only
• Build time delta: none, instruction-only
• Lighthouse delta: none, instruction-only
• API latency delta: none, instruction-only
• DB query delta: none, instruction-only

Risk / Notes

• Codex code review not requested because review quota/credits are exhausted.

Screenshots (UI only)

• Not applicable, instruction-only.

Lockfile rationale (if lockfile changed)

• Not applicable, no lockfiles changed.