Define a reproducible migration evidence gate

[tuanle96]

Outcome

Replace the ambiguous production_grade snapshot gate wording from #27 with a concrete, reproducible migration evidence bundle that tells a consultant what is safe, what needs review, and what needs a migration script.

The action taxonomy (no_action / needs_review / needs_script) already shipped in v1.1.0. What is missing is a defined artifact and a mechanical pass/fail interpretation.

Proposed artifact

One versioned JSON evidence bundle containing:

• source and target Odoo versions;
• addon/source identifiers and scan timestamp;
• data-quality summary with ACL/coverage caveats;
• upgrade-log findings and schema/source worklist;
• unresolved needs_review and needs_script items;
• tool/package version and reproducibility metadata;
• explicit gate result with machine-readable reasons.

Acceptance criteria

• JSON schema defines required fields, bounds, versioning, and allowed gate outcomes.
• At least one clean fixture and one blocked fixture demonstrate interpretation.
• The gate fails only on documented invariants; it never equates incomplete ACL visibility with clean data.
• Existing data_quality_report, analyze_upgrade_log, scan_addons_source, and upgrade_risk_report outputs map into the artifact without a second competing taxonomy.
• A human-readable guide explains what the artifact proves and what still requires consultant judgment.
• Focused regression tests, structural test, architecture fitness, Ruff, and mypy pass.

Decision required before implementation

Choose either:

1. adopt the evidence-bundle definition above under a descriptive name such as migration_readiness_evidence; or
2. drop the old production_grade label entirely if no consumer needs a hard gate.

This issue is definition-first. It does not authorize automatic remediation or direct writes.