feat: add atomic PR check workflow

[yogeshchoudhary147]

Summary

Adds a GitHub Actions workflow that automatically analyses every PR targeting main for atomicity signals and posts a structured comment with the findings.

How it works

On every PR open, edit, or push, the workflow runs the following checks:

Violations (fails the check):

• PR title contains "and"
• Files changed >= 40
• Total lines changed >= 1500

Warnings (advisory, check stays green):

• Files changed >= 15
• Total lines changed >= 400
• Renames or moves mixed with logic changes
• Formatting or config files mixed with source changes
• Changes spread across more than 3 unrelated top-level areas

The bot posts a single comment on the PR and updates it on every subsequent push, no duplicate spam.

What is not changed

Branch protection rules are untouched. Nothing is enforced until the team decides to flip that switch.

Test plan

• Raise a PR with "and" in the title, check should fail
• Raise a clean, single-concern PR, check should pass
• Push a new commit to an existing PR, bot comment should update not create a duplicate

[github-actions]

Atomic PR Check — Passed ✓

No atomicity concerns found. This PR looks well-scoped.