PlatyPS 2c: migrate Build-PSBuildMAMLHelp to Export-MamlCommandHelp

[tablackburn]

Part of #120 (Phase 2 — PlatyPS migration, #105). Blocked by the markdown migration — MAML is generated from the markdown this chain produces, so the schema has to settle first.

Scope

Migrate PowerShellBuild/Public/Build-PSBuildMAMLHelp.ps1 from New-ExternalHelp to Import-MarkdownCommandHelp | Export-MamlCommandHelp.

Closes #100 (Build-PSBuildMAMLHelp tests) — written against the new API.

Watch for

• The new pipeline is two cmdlets where the old was one; error handling on a malformed markdown file needs to fail the build rather than silently emit partial MAML
• Verify the emitted MAML is loadable: Get-Help against the built module should return correct syntax, parameter, and example blocks — the 5.1 validation on [Tracking] PowerShellBuild v1.0.0 roadmap #120 confirmed the pipeline produces parseable MAML, but not that this repo's docs survive it

Done when

Build-PSBuildMAMLHelp uses only 1.x cmdlets, #100's tests cover it, Get-Help output is verified end-to-end, and any behavior change has a migration-guide entry.