Move ThreatCrush CI workflow into .github/workflows/

[ralyodio]

Relocates the workflow staged at docs/threatcrush-scan.yml in #2 into .github/workflows/, where GitHub Actions actually reads it. No content change.

Completes PRD goal G2. First run is expected to FAIL: cli.threatcrush.com does not resolve, so the job writes an empty SARIF, reports that the scanner did not run, and exits non-zero rather than showing a green check that means nothing.

🤖 Generated with Claude Code

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

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[github-actions]

ThreatCrush scan results

0 finding(s) from threatcrush.

Severity	Count
—	0

Detection coverage — FAIL

Metric	Result
True positive rate	0.0% (0/77)
False positive rate	0.0% (0/39)
Missed detections	77

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Every file under vulns/ contains intentional vulnerabilities used to
measure scanner accuracy. Findings there are the expected outcome, not a
regression. See docs/VULNERABILITY_CATALOG.md.

_{Posted by scripts/generate-report.py.}