Strengthen validation and community health

[vartaxe]

Summary

Upgrade the Windows PowerShell 5.1 validation suite from Pester 5.7.1 to the exact Pester 6.2.0 stable pin after compatibility proof, and add project-specific contributing, conduct, issue, and pull request guidance. No runtime disk-layout code changes.

Safety impact

• The fail-closed target-selection and pre-clean guards are unchanged.
• No support boundary, destructive behavior, parameter, or task-sequence requirement changed.
• Community templates explicitly prohibit sensitive deployment details and require disposable-media handling for destructive tests.

Validation

• powershell.exe -NoProfile -ExecutionPolicy Bypass -File .\build\Invoke-Validation.ps1 passes — 63 passed, 0 failed, 0 skipped, 0 not run under installed Pester 6.2.0 on Windows PowerShell 5.1.
• Root and packaged script copies remain byte-identical — enforced by the passing validator.
• New or changed logic has synthetic tests that do not modify hardware — no runtime logic changed; existing synthetic suite passed unchanged.
• Documentation reflects changed parameters, integration, or compatibility — validation pin and test prerequisite updated; runtime documentation unchanged because behavior did not change.
• Destructive behavior was tested only on disposable media, or the untested hardware boundary is stated explicitly — no destructive test was performed; existing non-field-certified/disposable-hardware boundary remains explicit.

Additional checks: all five repository YAML files parsed successfully and git diff --check passed.

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