fix(gate): harden reviewer contract diagnostics

[screenleon]

Summary

• enforce and document valid severity/origin combinations for blocking reviewer findings
• emit precise, terminal-safe diagnostics from the shared verifier and its generated fallback
• add mutation-sensitive regressions for medium-severity blockers, pre-existing origins, synthesis short-circuiting, and JSON-escaped terminal control characters

Root cause

The reviewer-result contract rejected invalid blocking combinations through a generic finding-contract failure, which made remediation unnecessarily opaque. The first field-level diagnostics also reflected reviewer-controlled JSON values through raw tostring, leaving a terminal/log control-character injection path, while the new origin-specific branch lacked direct regression coverage.

Impact

Malformed reviewer artifacts now fail closed with actionable field-level diagnostics. Every reflected reviewer value is JSON-escaped before reaching stderr, so control characters remain inert. Valid reviewer results and the existing verifier/fallback ownership boundary are unchanged.

Validation

• bash tests/shell/test-pr-gate.sh --filter reviewer-protocol — 16 passed, 0 failed
• tests/bin/run-tests.sh --base main — 8 passed, 0 failed, 0 skipped
• Claude targeted remediation PR Gate — Final: GO; QA and Security approved with zero findings
• tests/bin/run-all-tests.sh — 97 passed, 0 failed, 0 skipped
• authoritative full-result verification passed for the committed tree