Add contracts pre-PR checklist

[kitWarse]

Summary

• Adds a contracts-specific pre-PR checklist to the pull request template so contributors have a concrete review-readiness guide before opening changes.
• The checklist calls out the expected verification commands, test coverage expectations, storage-layout safety review, event compatibility review, documentation updates, and scope hygiene for this contracts repo.

Issue

• Closes Add docs for contributor checklist before opening a contracts pull request #372

Changes

• Expanded .github/PULL_REQUEST_TEMPLATE.md with actionable checklist items for creator-keys tests and failure paths.
• Added explicit checklist coverage for cargo fmt, cargo clippy, and cargo test validation.
• Added storage safety and event compatibility reminders linked to the existing contracts documentation.
• Clarified that unrelated formatting, lockfile, generated artifact, and dependency changes should stay out of scoped contract PRs.

Validation

• Ran git diff --check upstream/main...HEAD.
• Did not run cargo fmt, clippy, or tests because this is a Markdown-only PR template update with no Rust code changes.

Notes

• Fork relationship verified: kitWarse/accesslayer-contracts is a fork of accesslayerorg/accesslayer-contracts.
• Branch was rebased onto the latest fetched upstream/main before pushing.

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