Add GitHub workflow updates and a PULL_REQUEST_TEMPLATE like leanSpec

[dicethedev]

Description

leanSig currently has no pull request template. Contributors opening PRs have no structured prompt for describing what changed, why, and how it was tested leading to inconsistent PR descriptions.

Motivation

This would help make PRs more consistent, reduce missing context in reviews, and remind contributors to run checks and consider tests/docs before opening a PR.

Proposal

Add a .github/PULL_REQUEST_TEMPLATE.md modelled after the one used in leanSpec,
covering at minimum:

• 🗒️ Description / Motivation - what the PR addresses
• What Changed - files touched and a brief summary of each
• Correctness / Behavior Guarantees - any invariants preserved or broken
• Tests Added / Run - what was tested and how
• ** 🔗 Related Issues or PRs** - Closes #N links

If this sounds good, I’d be happy to open a PR for it.

[dicethedev]

@tcoratger @TomWambsgans