review: add severity exit threshold

[efegokdemir]

Summary

Fixes #9 by adding an opt-in severity gate to boro review while preserving the existing default exit behavior. The JSON output already carries schema_version: 1; this PR documents that contract alongside the new CI/pre-send integration option.

Changes

• Add review --fail-on=critical|high|medium|low.
• Return exit status 1 when surviving findings meet or exceed the selected threshold.
• Evaluate validated_findings when available, otherwise use final findings.
• Document severity gating and the versioned JSON contract.
• Add CLI parsing and threshold-selection tests.

Testing

• cargo fmt -- --check — passed.
• cargo test cli_tests — 20 passed.
• cargo test — 451 unit tests passed; integration tests had 4 passed and 1 pre-existing failure in apply_message_id_handles_one_patch due to its existing quoted-path expectation.
• cargo test --test apply_range_cli apply_single_commit_keeps_schema_v1 — passed.
• cargo clippy --all-targets --all-features -- -D warnings — blocked by 19 pre-existing warnings in unrelated files; no warning was reported for the changed code.

Notes

The threshold is intentionally opt-in and applies after final validation, so it does not change existing review results or default exit codes. --json output is still emitted before the non-zero exit status, making the result suitable for CI artifact capture.

This contribution was prepared with AI assistance and manually checked against the repository's source, tests, and command output.