Add PR preview pruning and retention management

[Yashisinghal285]

Description

This PR adds preview pruning and retention management for PR preview deployments.

What changed

• Adds PREVIEW_RETENTION_LIMIT = 6
• Implements pruning of old pr-preview/pr-* directories
• Uses sparse checkout for efficient gh-pages maintenance
• Ensures only 6 most recent previews are kept
• Adds notifications when previews are pruned
• Preserves cleanup-on-close behavior

Issue

Fixes #377

Notes for Reviewers

• Workflow runs after PR preview deployment
• Only gh-pages pr-preview directory is modified
• Safe for concurrent PR workflows

Checklist

• I tested the workflow locally / validated syntax
• I understand this modifies GitHub Actions behavior

Signed commits

• Yes, I signed my commits

[github-actions]

🚀 Preview deployment: https://layer5io.github.io/getnighthawk/pr-preview/pr-378/

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[Yashisinghal285]

Hi maintainers 👋
This PR implements preview pruning and retention management as per #377.
Ready for review when convenient.