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>

commands/gsd/secure-phase.md

@@ -0,0 +1,35 @@
+---
+name: gsd:secure-phase
+description: Retroactively verify threat mitigations for a completed phase
+argument-hint: "[phase number]"
+allowed-tools:
+  - Read
+  - Write
+  - Edit
+  - Bash
+  - Glob
+  - Grep
+  - Task
+  - AskUserQuestion
+---
+<objective>
+Verify threat mitigations for a completed phase. Three states:
+- (A) SECURITY.md exists — audit and verify mitigations
+- (B) No SECURITY.md, PLAN.md with threat model exists — run from artifacts
+- (C) Phase not executed — exit with guidance
+
+Output: updated SECURITY.md.
+</objective>
+
+<execution_context>
+@~/.claude/get-shit-done/workflows/secure-phase.md
+</execution_context>
+
+<context>
+Phase: $ARGUMENTS — optional, defaults to last completed phase.
+</context>
+
+<process>
+Execute @~/.claude/get-shit-done/workflows/secure-phase.md.
+Preserve all workflow gates.
+</process>