Replace branch-mirror-status prompt file with PowerShell script

[lbussell]

There were two problems with branch-mirror-status prompt file introduced in #53902.

1. Prompt files are VS/VS Code only, so not useful for GitHub Copilot CLI.
2. Everything in the prompt is completely deterministic. It could be a script instead. So I did that.

Evaluation

	Before	After	Change
Execution time	84.7 seconds	7.56 seconds	−77.14 seconds (−91.1%)
Tokens used (Opus 4.6)	↑ 186.7k • ↓ 6.3k • 134.9k (cached)	0	−100%

Example Output

> ./eng/Get-BranchMirrorStatus.ps1
## Branch Mirror Status

| Repo | Branch | Latest GH Commit | Mirrored? | Details |
|---|---|---|---|---|
| dotnet/sdk | release/8.0.1xx | `cc809cd` [release/8.0.1xx] Update dependencies from dotnet/templat... | ✅ | Direct push (same SHA on AzDo branch) |
| dotnet/sdk | release/8.0.4xx | `55801ad` [release/8.0.4xx] Update dependencies from dotnet/source-... | ✅ | Direct push (same SHA on AzDo branch) |
| dotnet/sdk | release/9.0.1xx | `2797529` Replace ubuntu22 VM and Helix images with Azure Linux 3 (... | ✅ | Direct push (same SHA on AzDo branch) |
| dotnet/sdk | release/9.0.3xx | `a041d5f` [automated] Merge branch 'release/9.0.1xx' => 'release/9.... | ✅ | Direct push (same SHA on AzDo branch) |
| dotnet/installer | release/8.0.1xx | `c58a9a9` Replace ubuntu22 VM and Helix images with Azure Linux 3 (... | ✅ | Direct push (same SHA on AzDo branch) |
| dotnet/installer | release/8.0.4xx | `a77a3ff` [automated] Merge branch 'release/8.0.1xx' => 'release/8.... | ✅ | Direct push (same SHA on AzDo branch) |

[Copilot]

Pull request overview

This PR replaces the previously added branch-mirror-status Copilot prompt file with a deterministic PowerShell script that checks whether public GitHub release branches are mirrored to the corresponding internal Azure DevOps branches, producing markdown output suitable for sharing.

Changes:

• Added eng/Get-BranchMirrorStatus.ps1 to query GitHub (via gh) and AzDo (via az rest) and emit a markdown status table.
• Removed the Copilot prompt file .github/prompts/branch-mirror-status.prompt.md since prompt files are editor-scoped and the logic is scriptable.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
eng/Get-BranchMirrorStatus.ps1	Implements the branch mirror check and markdown reporting using gh + az rest.
.github/prompts/branch-mirror-status.prompt.md	Removes the prior editor-only prompt file now superseded by the script.

[marcpopMSFT]

in testing this locally, I always hit this error. I'm logged into az already (that experience isn't great btw). I have the az devops extension installed already, and az devops project list lists dnceng. What am I missing?

[lbussell]

Let me try on a different box and get back to you.