🚚 Move to pspete.Build GitHub Actions pipeline

[pspete]

Moves the module from AppVeyor to the shared pspete.Build pipeline (GitHub Actions, as in GhCID).

• appveyor.yml and the tracked build/ scripts are removed; build/ is gitignored and cloned from pspete.Build v1 by each job. The AppVeyor project is disabled.
• The psm1 dev loader is wrapped in #region Loader. The argument completer registrations move from Private/SIAArgumentCompleter.ps1 into the psm1, below the IdentityCommand helper copy, so they run after the helpers exist in the built module.
• RequiredModules: IdentityCommand 0.7.159. CI installs it in the test, test-module and release jobs.
• Help: the committed platyPS 0.14 xml is kept; the Update External Help steps are commented out.
• Tests/IdentityCommand.SIA.Tests.ps1 keeps only the module-specific contexts (Secure Value Handling, Shared Helpers).
• CHANGELOG: released headings converted to ## [x.y.z] - yyyy-MM-dd (## [x.y] - ... for per-minor sections), as version.ps1 and deploy-github expect.
• docs/collections/_posts/: backdated release notes posts, one per past major.minor, generated with pspete.Build release-notes.ps1 from the CHANGELOG history. Future posts are written by the release.
• New-SIAPolicyProviderDefinition help: -vnetIds gets a description (it held a platyPS placeholder, which the help test rejects), and -vpcIds describes AWS/GCP VPC IDs.

PR CI does not run for dev; the full pipeline first runs on the dev -> main PR. Merging that releases 0.6.68 (patch bump from the Unreleased ### Changed entries) to GitHub and the PowerShell Gallery.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.77%. Comparing base (db47e9c) to head (77bcc1d).
⚠️ Report is 17 commits behind head on dev.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #14      +/-   ##
==========================================
+ Coverage   92.64%   98.77%   +6.13%     
==========================================
  Files          50       51       +1     
  Lines         598      571      -27     
==========================================
+ Hits          554      564      +10     
+ Misses         44        7      -37     

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