Align feature request template

[PurpleSentinel]

What changed

• replace the generic GitHub feature-request prompts with the implementation-ready format used in recent Track Session Timer issues
• automatically apply the NewFeature label
• add guidance for action-oriented titles, user experience, functional behavior, hardware/safety constraints, acceptance criteria, documentation, and roadmap relationships

Why

Recent feature work uses a consistent structure that captures observable behavior, configuration and compatibility requirements, host coverage, physical-device validation, and parent/epic relationships. The repository template did not reflect that working format.

Impact

New feature issues will start with the same level of structure used by recent development issues such as #55, #57, and #59, reducing follow-up work before implementation.

Validation

• git diff --check
• confirmed the change is limited to .github/ISSUE_TEMPLATE/feature_request.md