Add PR card UI with mock data

[lucaslyl]

Summary

• Blocked by Cs 10217 trigger command on webhook event #4074 (real PR data integration). Creating this PR card ahead of time with mock data so the UI can be reviewed and iterated on independently.
• Adds a PrCard card type with isolated and fitted templates, styled using a github-pr-brand-guide.json Theme card.

What's Included

• pr-card.gts: PrCard card definition with fields (prTitle, prNumber, prUrl, submittedBy, submittedAt, listing)
• github-pr-brand-guide.json: GitHub-themed color palette as a Boxel Theme card
• Sample instance JSON wired to a demo owner

Demo:

Isolated:

Fitted:

[github-actions]

Host Test Results

    1 files  ±0      1 suites  ±0   1h 53m 25s ⏱️ + 1m 1s
1 915 tests ±0  1 900 ✅ +1  15 💤 ±0  0 ❌ ±0 
1 930 runs  ±0  1 915 ✅ +2  15 💤 ±0  0 ❌  - 1 

Results for commit d850a5d. ± Comparison against base commit 079e239.