[codex] Add release screenshot capture checklist

[jjoonleo]

Summary

• Adds a Google Play release screenshot capture guide for issue [Release] Capture release screenshots from real app screens #449 with setup, required screen coverage, Play asset constraints, quality checks, and an evidence log.
• Links the guide from the release checklist and docs home.
• Adds a scoped plan and handoff note because final screenshot capture remains manual.

Advances #449.
References parent track #466.

Verification

• git diff --check
• Documentation-only change; Flutter tests were not run.

Remaining human tasks

• Build and install the release candidate or equivalent signed build.
• Capture screenshots from real app screens for onboarding/permission, schedule creation, alarm/preparation, calendar/home, and My Page/account controls.
• Validate screenshot format and dimensions in Play Console.
• Have product/design approve the selected screenshot set.
• Complete the evidence log before closing [Release] Capture release screenshots from real app screens #449.