ci: cancel superseded PR runs of checks and e2e

[m4ttheweric]

• adds a concurrency group to checks.yml and e2e.yml so a newer push to a PR cancels its older run and frees the macOS slot (the org is on the free plan, 5 concurrent macOS jobs).
• main runs are never cancelled. purity.yml is left alone since it finishes in seconds.

🤖 Generated with Claude Code

[coderabbitai]

Warning

Review limit reached

Next included review available in 48 minutes.

Check out review usage here.

View limit details

Limit details: You’ve used the included review currently available. Your 86 included PR review attempts over the past 7 days set your current allowance at 1 review per hour.

Your organization has reached its usage spending cap. Adjust your spending cap in the billing tab.

Learn how review limits work.

Review configuration:

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: b6faea3f-4a24-44ac-a217-b19be7456a35

📥 Commits

Reviewing files that changed from the base of the PR and between 1d37692 and b62b1fb.

📒 Files selected for processing (2)

• .github/workflows/checks.yml
• .github/workflows/e2e.yml

---

_{Comment @coderabbitai help to get the list of available commands.}