chore(repo): add pull request template

[Adiz4415]

Summary

Adds a default PR description template at .github/pull_request_template.md so future contributors have a consistent structure for describing changes, test plans, and breaking-note callouts. The repo currently has no PR template; this file is auto-discovered by GitHub.

Changes

• Adds .github/pull_request_template.md with sections: Summary, Related Issues, Changes, Test Plan / Verification, Screenshots / Recordings, Checklist, Breaking Changes, and Notes for Reviewers.
• Style modeled on the existing .github/ISSUE_TEMPLATE/feature_request.md.
• Acknowledges both Conventional Commits scopes (e.g. feat(frontend): …) and bare types (fix: …, chore: …, docs: …) seen in recent history.
• Notes that changesets are added at the repo root via pnpm changeset (monorepo bucketing is automatic).
• Lints the template against both pnpm lint and cargo clippy since the monorepo has two workspaces.

Test Plan / Verification

• Markdown renders correctly when previewing the PR description on GitHub.
• Sections list is exhaustive enough for typical frontend (app/), smart-contract (contracts/), and docs-only PRs.
• No code, schema, or configuration was modified — this is a documentation-only change with no runtime impact.

Checklist

• PR title follows Conventional Commits
• No user-facing behavior changed — no changeset needed
• No code changes — no tests required
• No new lint warnings
• No breaking changes

Breaking Changes

None.