docs(16-01): complete threat intelligence sources plan

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.planning/STATE.md

@@ -3,14 +3,14 @@ gsd_state_version: 1.0
 milestone: v1.0
 milestone_name: milestone
 status: executing
-stopped_at: Completed 15-03-PLAN.md
-last_updated: "2026-04-06T13:37:48.053Z"
+stopped_at: Completed 16-01-PLAN.md
+last_updated: "2026-04-06T13:46:09.387Z"
 last_activity: 2026-04-06
 progress:
   total_phases: 18
   completed_phases: 14
   total_plans: 81
-  completed_plans: 80
+  completed_plans: 81
   percent: 20
 ---
 
@@ -99,6 +99,7 @@ Progress: [██░░░░░░░░] 20%
 | Phase 14 P01 | 4min | 1 tasks | 14 files |
 | Phase 15 P01 | 3min | 2 tasks | 13 files |
 | Phase 15 P03 | 4min | 2 tasks | 11 files |
+| Phase 16 P01 | 4min | 2 tasks | 6 files |
 
 ## Accumulated Context
 
@@ -148,6 +149,9 @@ Recent decisions affecting current work:
 - [Phase 14]: RegisterAll extended to 45 sources (40 Phase 10-13 + 5 Phase 14 CI/CD); CircleCI gets dedicated CIRCLECI_TOKEN
 - [Phase 15]: Discord/Slack use dorking approach (configurable search endpoint) since neither has public message search API
 - [Phase 15]: Log aggregator sources are credentialless, targeting exposed instances
+- [Phase 16]: VT uses x-apikey header per official API v3 spec
+- [Phase 16]: IX uses three-step flow: POST search, GET results, GET file content
+- [Phase 16]: URLhaus tag lookup with payload endpoint fallback
 
 ### Pending Todos
 
@@ -162,6 +166,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-04-06T13:32:52.610Z
-Stopped at: Completed 15-03-PLAN.md
+Last session: 2026-04-06T13:46:09.383Z
+Stopped at: Completed 16-01-PLAN.md
 Resume file: None