fix: keep severity calibration consistent across scan modes

[cjc0013]

Summary

Deep reduction could silently assign a different severity from Standard scan findings even when the underlying evidence was materially equivalent. This change preserves equivalent calibration and requires evidence-backed explanations when a reducer changes or reconciles severity.

Closes #671.

Changes

• compare each Deep reduction severity with every claimed Standard source finding
• require both severity.rationale and severity.changeConditions when source severities conflict or the output changes their calibration
• clarify in the reducer prompt that severity is evidence-driven rather than mode-, worker-, repetition-, or wording-driven
• add regression coverage for silent recalibration, conflicting source severities, and an explicitly justified reconciliation

Validation:

Passed:

• cmd.exe /d /c pnpm --dir sdk/typescript run build:plugin
• cmd.exe /d /c node plugins/codex-security/mcp-app/tests/test_artifact_deep_reducer.mjs
• pnpm --dir sdk/typescript run test:mcp
• npm --prefix plugins/codex-security/mcp-app run typecheck
• python -m ruff check --config plugins/codex-security/pyproject.toml .github/scripts/check_plugin_source_compatibility.py .github/scripts/test_check_plugin_source_compatibility.py plugins/codex-security
• python -m ruff format --check --config plugins/codex-security/pyproject.toml .github/scripts/check_plugin_source_compatibility.py .github/scripts/test_check_plugin_source_compatibility.py plugins/codex-security
• python .github/scripts/check_plugin_source_compatibility.py

Risk and rollout

The new check applies only after Deep reduction has retained and validated source-finding provenance. Existing reductions that preserve a unanimous source severity are unchanged. A reducer that changes severity or resolves conflicting source severities without the required explanation now fails closed and can be retried with the missing evidence fields.

Prior work and attribution

• soyeon-oai authored issue Keep severity calibration consistent across scan modes #671, documented the cross-mode calibration inconsistency, and defined the expected evidence-backed behavior.
• su-jin1425 expressed interest in investigating the issue and explicitly requested confirmation before starting. No patch, diagnosis, design, or code from that comment was used.

Public disclosure review

• No customer, partner, prospect, or user identities, data, or identifying details are included.
• No credentials, personal data, private source, scan findings, or nonpublic links or tickets are included.
• I reviewed the branch name, title, description, commits, changes, comments, logs, screenshots, attachments, and links for public disclosure.