Add Copilot prompt guide for backporting PRs to release branches

This PR introduces a comprehensive guide for GitHub Copilot to assist with backporting changes to PowerShell release branches. The prompt provides step-by-step instructions, best practices, and automated workflows for creating backport PRs.

What's Added

New File: .github/prompts/backport-pr-to-release-branch.prompt.md

• 536 lines of detailed guidance for backporting PRs
• Structured workflow with 6 distinct steps
• Real-world examples and troubleshooting guidance

Key Features

1. Automated PR Discovery

• Search for PRs that need backporting using labels
• Filter by version and triage state (Consider, Approved, Done)
• Present sorted lists to users for selection

2. Pre-flight Checks

• Verify PR is merged before attempting backport
• Check for existing backport PRs to avoid duplicates
• Interpret backport labels (Migrated, Done, Approved, Consider)
• Extract PR metadata (merge commit SHA, author, CL labels)

3. Smart Conflict Resolution

• Fetch and analyze original PR diffs before resolving conflicts
• Identify conflict types (parameter changes, code removal, refactoring)
• Context-aware resolution strategies that preserve release branch specifics
• Automatic conflict summary generation for user review
• Real-world example from PR Remove usage of fpm for DEB package generation #26281 showing parameter set handling

4. Structured PR Creation

• Standardized title format: [release/v7.x] <original-title>
• Complete PR body template with required sections:
  • Backport reference with auto-generated metadata
  • Impact assessment (Tooling vs Customer)
  • Regression information
  • Testing approach
  • Risk evaluation (High/Medium/Low with definitions)
• Detailed guidelines for filling out each section

5. Manual Backport Tools

• Documentation for Invoke-PRBackport PowerShell function
• Bulk backporting with Invoke-PRBackportApproved
• Backport reporting tools
• Branch naming conventions (automated vs manual)

6. Cleanup and Validation

• Automatic cleanup of temporary diff files
• Comprehensive "Definition of Done" checklist
• Best practices for testing and CI validation

Benefits

• Consistency: Ensures all backport PRs follow the same format and include required information
• Efficiency: Automates repetitive tasks like fetching PR info, creating branches, and formatting PR descriptions
• Quality: Provides conflict resolution strategies and requires documentation of changes
• Discoverability: Helps identify PRs that need backporting and checks for duplicates
• Knowledge Sharing: Documents the backport process for both AI assistants and human maintainers

Testing

This prompt was tested by successfully backporting PR #26281 to the release/v7.4 branch, which included:

• Handling merge conflicts in build.psm1
• Resolving parameter set differences between main and release branches
• Creating properly formatted backport PR [release/v7.4] Remove usage of fpm for DEB package generation #26382
• Documenting conflict resolution approach

Related Work

• Complements existing PowerShell release tools in tools/releaseTools.psm1
• Aligns with documented release processes in docs/maintainers/releasing.md
• Supports the existing automated backport bot (pwshBot) workflow

Note: This is a Copilot prompt file (.prompt.md) that GitHub Copilot can use when users ask for help with backporting. It doesn't affect the codebase directly but provides structured guidance for AI-assisted backporting tasks.