feat: add security-first enforcement layer with threat-model-anchored verification

Adds /gsd:secure-phase command and gsd-security-auditor agent as a
threat-model-anchored security gate parallel to Nyquist validation.

New files:
- agents/gsd-security-auditor.md — verifies PLAN.md threat mitigations
  exist in implemented code; SECURED/OPEN_THREATS/ESCALATE returns
- commands/gsd/secure-phase.md — retroactive command, mirrors validate-phase
- get-shit-done/workflows/secure-phase.md — enforcing gate: threats_open > 0
  blocks phase advancement; accepted risks log prevents resurface
- get-shit-done/templates/SECURITY.md — per-phase threat register artifact

Modified:
- config.json — security_enforcement (absent=enabled), security_asvs_level,
  security_block_on parallel to nyquist_validation pattern
- VALIDATION.md — Threat Ref + Secure Behavior columns in verification map
- gsd-planner.md — <threat_model> block in PLAN.md format + quality gate
- gsd-executor.md — Rule 2 threat model reference + ## Threat Flags scan
- gsd-phase-researcher.md — ## Security Domain mandatory research section
- plan-phase.md — step 5.55 Security Threat Model Gate
- execute-phase.md — security gate announcement in aggregate step
- verify-work.md — /gsd:secure-phase surfaced in completion routing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

get-shit-done/workflows/execute-phase.md

@@ -436,6 +436,27 @@ After all waves:
 ### Issues Encountered
 [Aggregate from SUMMARYs, or "None"]
 ```
+
+**Security gate check:**
+```bash
+SECURITY_CFG=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow.security_enforcement --raw 2>/dev/null || echo "true")
+SECURITY_FILE=$(ls "${PHASE_DIR}"/*-SECURITY.md 2>/dev/null | head -1)
+```
+
+If `SECURITY_CFG` is `false`: skip.
+
+If `SECURITY_CFG` is `true` AND `SECURITY_FILE` is empty (no SECURITY.md yet):
+Include in the next-steps routing output:
+```
+⚠ Security enforcement enabled — run before advancing:
+  /gsd:secure-phase {PHASE} ${GSD_WS}
+```
+
+If `SECURITY_CFG` is `true` AND SECURITY.md exists: check frontmatter `threats_open`. If > 0:
+```
+⚠ Security gate: {threats_open} threats open
+  /gsd:secure-phase {PHASE} — resolve before advancing
+```
 </step>
 
 <step name="handle_partial_wave_execution">