Tighten PR template for issue and proof traceability

[ExplodingUFO]

Problem

The current pull request template is too sparse for the repository's issue-PR-driven workflow. It asks for summary, docs, and checks, but it does not explicitly force contributors to name the linked GitHub issue, Beads issue, review-gate status, UI/screenshot impact, or exact verification evidence.

This makes small governance/doc changes and larger React Flow parity work harder to review because the reviewer has to infer whether the PR followed the required issue -> bead -> worktree -> proof path.

Scope

Update only .github/pull_request_template.md so every PR has a lightweight, grep-friendly checklist for:

• linked GitHub issue and Beads issue
• isolated worktree/branch provenance
• verification commands or explicit docs-only rationale
• UI/screenshot impact classification
• compatibility/migration/public API impact
• review-gate / merge status acknowledgement

Do not touch product code, active React Flow parity docs, serialization docs, or existing open PR files.

Acceptance Criteria

• The PR template contains fields/checklist items for GitHub issue, Beads issue, worktree/branch, verification, UI/screenshot impact, compatibility/migration/API impact, and review-gate status.
• The wording stays concise enough that routine small PRs are not burdened with long prose.
• The change is isolated to .github/pull_request_template.md.
• Whitespace/Markdown sanity checks pass.

Parallelism / Dependencies

Independent of PR #200, PR #261, and PR #263 because it touches a separate GitHub template file. No dependency on the blocked whiteboard stack.