docs(06): create phase 6 plan — performance & production hardening

6 plans across 4 waves covering structlog/Loki observability, Locust
load testing, multi-stage Dockerfile hardening, trusted-proxy rate
limiting, and RUNBOOK.md. Verification passed (0 blockers).

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

.planning/STATE.md

@@ -1,16 +1,16 @@
 ---
 gsd_state_version: 1.0
 milestone: v1.0
-milestone_name: "audit gaps: SHARE-02/STORE-06/ADMIN-06"
-current_phase: 06.2
-status: executing
-last_updated: "2026-06-02T18:24:12.300Z"
+milestone_name: "Performance & Production Hardening"
+current_phase: "06"
+status: planned
+last_updated: "2026-06-02T00:00:00.000Z"
 progress:
-  total_phases: 3
-  completed_phases: 2
-  total_plans: 7
-  completed_plans: 7
-  percent: 67
+  total_phases: 1
+  completed_phases: 0
+  total_plans: 6
+  completed_plans: 0
+  percent: 0
 ---
 
 # Project State