feat: enhance GitHub PR resource with automatic build URLs

[ujala-singh]

Add comprehensive enhancements to improve integration with Concourse CI:

1. Automatic Build URL Generation:

   • UpdateCommitStatus now auto-generates Concourse build URL when target_url is empty
   • Constructs URL from ATC_EXTERNAL_URL and BUILD_ID environment variables
   • Enables clickable GitHub status checks linking back to pipeline builds

2. Environment Variable Expansion:

   • Add safeExpandEnv() function for secure environment variable expansion
   • Supports Concourse build metadata variables: BUILD_ID, BUILD_NAME, BUILD_JOB_NAME, BUILD_PIPELINE_NAME, BUILD_TEAM_NAME, ATC_EXTERNAL_URL
   • Applied to target_url, context, and comment fields for dynamic values

3. Enhanced Configuration Parameters:

   • Add base_context support for customizable status check context prefix
   • Add description_file support for reading status description from file
   • Fix delete_previous_comments type from string to boolean

4. Comment Management:

   • Implement DeletePreviousComments() using GitHub GraphQL + REST API
   • Queries user's previous comments via V4 API
   • Deletes matching comments via V3 API to keep PR conversations clean

Changes:

• models/github.go: Auto URL generation, DeletePreviousComments()
• pr/out.go: safeExpandEnv(), enhanced parameter handling
• pr/models.go: Updated OutParams with new fields
• pr/helpers_test.go: Fixed test for boolean delete_previous_comments

All 51 tests passing with improved coverage.