feat: add grenier net-exit method

[moul]

Linked board issue: https://github.com/1789-tech/job-board/issues/67

Summary

• adds references/method.md with the net-exit-value doctrine, priority-aware 20/20/20 cutoff, five object classes, keep veto, deadline policy, receipt derivation contract, and Vinted draft-safety workflow
• wires the method into SKILL.md, pricing.md, disposition.md, listing.md, the FR country recipe, README, examples, and eval fixtures
• adds a method-delta example that shows the expected grenier output while explicitly refusing to invent missing Claude-nu baselines

Verification

• make validate
• jq . evals/evals.json >/dev/null
• git diff --check
• Official FR source checks on 2026-07-06: Vinted how-it-works/help, Leboncoin rules, Vestiaire seller fees, Emmaus donation, ecosystem electronics, Refashion collection-point pages

Blocked / not complete

• make test cannot run in this environment because go is not installed (go: No such file or directory).
• The required same-day real Claude-nu baselines could not be captured because claude --bare -p --model opus ... returns Not logged in · Please run /login.

Draft until those two external checks are completed.