feat: add optional EvidentLoop audit integration

[Li-Sanze]

Summary

• Retire the default CrossReview execution surface while preserving a product-neutral Verifier contract.
• Add EvidentLoop as an officially recommended optional audit companion.
• Support optional installation across the existing host adapters without changing Sopify's default workflow.
• Align plan, writer, distribution, and installation contracts, then archive the completed plan with its audit evidence.

Why

Sopify needs a supported audit path without coupling its core workflow to one verifier or making external auditing mandatory.

Behavior

• The default Sopify installation remains unchanged.
• Existing healthy EvidentLoop installations are reused and are not upgraded automatically.
• Optional installation failures do not roll back Sopify or leave an unverified Skill directory.
• EvidentLoop remains optional and does not become Sopify's runtime dependency or sole verifier.

Validation

• Targeted tests: 55 passed, 48 subtests passed
• Protocol smoke tests: 5 passed
• EvidentLoop review: pass candidate with bound diff and report versions

Stack

The plain-language workflow and product-page changes are reviewed separately in the follow-up PR based on this branch.