Miapp: derive base branch from origin/HEAD instead of $env:RepoBranchName

[mazhelez]

Summary

Miapp (the MicroApp Integrate tooling under build/scripts/Miapp/) derived its base branch from $env:RepoBranchName. When Miapp runs inside a repository whose default branch differs from an outer/consuming repository, that ambient variable points at the wrong branch. In practice, a monorepo that vendors BCApps as a submodule sets RepoBranchName=master for its own repo, but BCApps' default branch is main. Miapp then constructed refs like origin/master, which do not exist in BCApps, and failed:

fatal: ambiguous argument 'HEAD..origin/master': unknown revision or path not in the working tree.

Change

• Replace Initialize-MiappRepoBranchName (env-based) with Get-MiappBaseBranch, which resolves the base branch from the repository's own origin/HEAD via git symbolic-ref --short refs/remotes/origin/HEAD, with git remote show origin as a fallback. The result is cached in a module-scoped variable.
• No environment variable is read or written, so Miapp always targets the default branch of the repository it operates on.
• Update all origin/$env:RepoBranchName usages across MicroApp.psm1, MicroAppGitHelper.psm1, MicroAppIntegrate.psm1, and MicroSnapApp.psm1, plus the README.

Testing

• All modules parse cleanly.
• Get-MiappBaseBranch returns main in BCApps even when $env:RepoBranchName=master.
• The two previously failing commands (git log HEAD..origin/<base> and git log origin/<base>..HEAD) now resolve origin/main and exit 0.

AB#643986