Validate PowerShell reviewer policy before updater mutations

[BaseInfinity]

Origin

Nonblocking P2 follow-up from the completed #143 exact-candidate review.

Problem

The PowerShell install/update path can read an existing cross-model reviewer value and begin mutation before proving that the value is canonical. A later validation failure still fails closed, but the repository may already contain a partial updater mutation.

The POSIX and PowerShell paths should have the same pre-mutation policy boundary.

Acceptance criteria

• Parse and validate any existing reviewer lane before the first filesystem mutation.
• Reject unsupported reviewer/model/effort combinations with no changed files.
• Preserve valid customized repository policy.
• Add a Windows-focused regression proving invalid input leaves the checkout byte-identical.
• Keep this outside 1.0 unless a real consumer reproduction raises the severity.

Boundary

Do not redesign reviewer lanes. This is a small updater atomicity/safety correction.