docs: add GitHub Actions CI integration examples

[dmchaledev]

Summary

• The README described the tool as a "CI gate" but only showed a bare one-liner (|| echo "Gate failed"), leaving users to figure out the workflow integration themselves
• Added a dedicated CI Integration section with three copy-pasteable GitHub Actions patterns

What's added

Pattern	Purpose
Blocking gate	Fails the workflow when grade is D or F — the primary use case
Informational audit	Always passes; useful during a remediation period or for visibility without blocking
JSON artifact export	Saves the full report as a downloadable workflow artifact for audit trails

Why this is high-leverage

The CI gate use case is explicitly called out in the README's "Who Is This For" section ("automated header auditing on every deployment"), but there was no example showing how to wire it up. A copy-pasteable workflow lowers the adoption barrier to near zero for the tool's primary audience (DevSecOps teams, security engineers).

Test plan

• Verify Markdown renders correctly (no broken code fences)
• Confirm the three YAML snippets are valid GitHub Actions syntax
• Spot-check that the || true pattern correctly suppresses the non-zero exit

https://claude.ai/code/session_01FV2i2kr45Q4ztmCtTz8xzv

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