Update feature_issue.md template with PRD structure

[rtibblesbot]

Summary

Replaces the existing feature issue template with a structured PRD (Product Requirements Document) format. The new template provides clear, focused sections for scoping and risk management.

Changes:

• Replaced Overview/Goals/User Stories sections with PRD structure: Requirements, Outcomes, Constraints, Dependencies, Assumptions, Risks, Timeline, Out of Scope, and Context (with Relevant Feedback, Technical Context, and Reference Links subsections)
• Removed Contributors, Target Quarter and Due date, and Product Issues sections
• Preserved frontmatter (name, about, title, labels, type) and AI usage section with DEEP guidelines

Verification: Compared the output template against the proposed template in the issue — all sections match exactly.

References

Closes #59

Reviewer guidance

This is a template-only change to .github/ISSUE_TEMPLATE/feature_issue.md. No code, tests, or CI affected.

To verify:

1. Compare the new template sections against the proposed template in Update feature_issue.md template to PRD structure #59
2. Confirm the frontmatter block (lines 1-8) is unchanged from main
3. Confirm the AI usage HTML comment and section (bottom of file) is preserved unchanged

AI usage

Claude Code was used to implement this template change by reading the issue requirements and applying the proposed template. The output was verified against the issue's proposed template and acceptance criteria.

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@rtibblesbot's comments are generated by an LLM, and should be evaluated accordingly

How was this generated?

• Ran pre-flight CI checks (lint, format, tests) and verified all pass
• Rebased onto the target branch and resolved any conflicts
• Reorganized commit history into clean, logical commits
• Audited the diff to ensure only issue-relevant files are changed
• Built PR body from the repository's PR template with evidence blocks

Closes #59