Remove US stamp from GitHub Status endpoints #580

Context

The module includes support for querying the GitHub Status API through three regional
"stamps": Public, Europe, and US. These stamps are defined in the module's initialization and used by the status
functions (Get-GitHubStatus, Get-GitHubScheduledMaintenance, Get-GitHubStatusComponent, Get-GitHubStatusIncident)
via the -Name parameter and Get-GitHubStamp.

Request

The US stamp (https://us.githubstatus.com) no longer exists. All API calls to this base URL return
404 (Not Found), causing 8 test failures in CI:

• Get-GitHubScheduledMaintenance — scheduled, active, and upcoming maintenance queries for US
• Get-GitHubStatus — all status and summary status queries for US
• Get-GitHubStatusComponent — component status query for US
• Get-GitHubStatusIncident — all incidents and unresolved incidents queries for US

Error: [-] Get-GitHubScheduledMaintenance - Gets scheduled maintenance for US 448ms (447ms|1ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubScheduledMaintenance - Gets active maintenance for US 318ms (317ms|0ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubScheduledMaintenance - Gets upcoming maintenance for US 329ms (328ms|0ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubStatus - Gets all status for US 341ms (340ms|1ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubStatus - Gets summary status for US 419ms (418ms|1ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubStatusComponent - Gets the status of GitHub components for US 248ms (247ms|1ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubStatusIncident - Gets the status of all GitHub incidents for US 255ms (254ms|1ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Error: [-] Get-GitHubStatusIncident - Gets the status of unresolved GitHub incidents for US 371ms (370ms|1ms)
  Expected no exception to be thrown, but an exception "Response status code does not indicate success: 404 (Not Found)." was thrown

Acceptance criteria

• The US stamp is no longer registered in the module
• All status functions continue to work for Public and Europe stamps
• CI tests pass without the 8 US-related test failures
• Tab-completion no longer offers US as a stamp name

Technical decisions

Root cause: The US stamp is defined in src/variables/private/GitHub.ps1 as
[GitHubStamp]::new('US', 'https://us.githubstatus.com'). The URL https://us.githubstatus.com is no longer served
by GitHub and returns 404 on all API endpoints.

Fix scope: Remove the single line that registers the US stamp. No changes are needed to the status functions
themselves — they dynamically resolve stamps via Get-GitHubStamp, so removing the stamp entry is sufficient. The
parametrized test in GitHub.Tests.ps1 iterates over Get-GitHubStamp output, so the US test cases will
automatically disappear once the stamp is removed.

No breaking change: The US stamp has been non-functional (404), so removing it does not break any working
functionality. Any user currently passing -Name US is already getting errors.

Implementation plan

Core changes

• Remove the [GitHubStamp]::new('US', 'https://us.githubstatus.com') line from src/variables/private/GitHub.ps1

Verification

• Confirm the parametrized status tests pass for Public and Europe stamps
• Confirm Get-GitHubStamp returns only Public and Europe