[codex] Add summary-json CI cookbook

[stacknil]

Brief Design Summary

This PR adds a docs-only CI cookbook for --summary-json PATH.

The cookbook explains the summary-only artifact as the same object as report.json["summary"], provides minimal command usage, and includes Python and PowerShell consumer snippets for lightweight CI consumption. It also documents compatibility notes and non-claims: optional summary.policy / summary.enrichment, no unchanged, no CVE scanner behavior, no dependency safety oracle, no hidden network access by default, and production PyPI remaining deferred.

No runtime behavior, workflows, package metadata, release tags, examples, JSON schema, Markdown/SARIF output behavior, or publishing status changed.

Files Changed

• tools/sbom-diff-and-risk/docs/summary-json-ci-cookbook.md
• tools/sbom-diff-and-risk/README.md
• tools/sbom-diff-and-risk/docs/report-schema.md
• tools/sbom-diff-and-risk/docs/reviewer-evidence-pack.md

Validation

• git diff --check passed, including the new cookbook file via intent-to-add.
• Confirmed relative link targets exist.
• Confirmed package version remains 0.6.0.
• Confirmed no workflow files changed.
• No Python tests run because this is docs-only.

Out of Scope

• No CLI behavior changes
• No JSON schema changes
• No examples changes
• No workflow changes
• No package version bump
• No tag or GitHub Release
• No PyPI/TestPyPI publishing
• No production PyPI workflow