Docs: update pull request template

[SilverSupplier]

Summary

• Update the repository pull request template to match the review format used by recent PRs.
• Add Unity EditMode/PlayMode and GitHub checks to the default verification checklist.
• Add scope, out-of-scope, risk, screenshots/playtest, and issue-link sections so future PR bodies keep review context visible.

Verification

• node scripts/validate-doc-state.mjs
• git diff --check
• npm test not run; template-only change.
• npm run build not run; template-only change.
• Unity EditMode tests not run; template-only change.
• Unity PlayMode tests not run; template-only change.

Notes

• Scope: PR template only.
• Out of scope / follow-up: existing open PR bodies were already normalized separately.
• Risk notes: low; this only affects default text shown when opening future PRs.

Screenshots / Playtest Notes

Not applicable; no UI/gameplay change.