feat: Phase V V1 pre-release audit gateway (PRG-) -- BRC+PCC+ECI gate before external sharing

[cschanhniem]

Summary

• Adds PRG- (Pre-Release Audit Gateway) schema: single gate asserting BRC+PCC+ECI all pass before external sharing
• REQUIRED_GATES: BRC/PCC/ECI (must all be present, exactly once each)
• VALID_GATE_STATUSES: pass/fail/not_evaluated
• VALID_RELEASE_VERDICTS: approved (all pass) / blocked (any fail)
• build() evaluates BRC approved, PCC grade A/B, ECI grade A/B
• GateResult: gate_type, artifact_id, status, details per gate
• dry_lab_only=True enforced; PRG- prefix validated
• 53 tests; BASELINE 10143→10206

Test plan

• 53 unit tests pass: constants, build happy paths, validate rejections, format output
• Regression test passes
• Smoke test: all-pass → approved; brc_verdict=blocked → blocked; pcc_grade=C → blocked