Issue Description

scripts/extension/Package-Extension.ps1 has a 129-line <script> entry point block that cannot be tested via dot-sourcing. This code handles path validation, version resolution, directory operations, vsce invocation, and cleanup.

Current state:

• File coverage: 27% line (55/204)
• Method coverage: 100% (7/7) — all pure functions are tested
• Entry point: 0% coverage (129 lines)

Acceptance criteria:

• Extract entry point logic into Invoke-PackageExtension function
• Keep <script> block as thin wrapper calling the new function
• Add comprehensive tests with mocked file system and vsce calls
• Test parameter validation, error paths, and cleanup logic
• Achieve ≥85% line coverage

Estimated coverage improvement: 27% → ~90%

Additional Context

This follows the refactoring pattern established in PR #221 ("extract pure functions for Pester testability"). The entry point contains business logic that should be testable:

• Version resolution from manifest
• Extension directory validation
• vsce command construction
• Output path handling

This is part of a broader effort to increase unit test coverage from ~42% to ~85-90%.