docs(phase-18): add security threat verification (SECURED, 0 open)
gsd-security-auditor verified all 13 plan-time STRIDE threats against the
implementation: 8 mitigate confirmed in code (file:line evidence), 5 accept
documented, 4 supply-chain checks (zero new deps). threats_open: 0. ASVS L1,
block_on high — no high-severity gaps. Post-review fixes (WR-01 blank-TZ guard,
WR-02 INSERT IGNORE) verified in code; D-07 boundary confirmed via git diff.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
