Scope reduction: remove rail/3D, focus on portal-frame pole 2D detection

- Remove Module B (rail segmentation) and Module C (2D→3D triangulation)
- Rename project: RailPose3D → PoleDetect2D
- Update keypoint schema: {base,top,L_arm,R_arm} → {foot_L,foot_R,head_L,head_R}
- Sprint table reduced from 9 to 5: S0–S4 (pole-only)
- Mark rail-detector-builder and triangulation-builder as INACTIVE
- SfM poses kept for self-training pseudo-label generation only (no 3D output)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/agents/rail-detector-builder.md

@@ -1,6 +1,6 @@
 ---
 name: rail-detector-builder
-description: RailPose3D Module B (레일 segmentation + polyline)의 Generator 에이전트. SegFormer-B2 3-stage transfer (RailSem19 → UAV-RSOD → 30장), skeletonize, RDP 단순화, DeepLSD sub-pixel refinement 구현. Module B 관련 sprint (S1 부분, S3, S6의 B 부분) 에서 호출.
+description: "[INACTIVE — 2026-04-28 범위 축소로 비활성화] 레일 segmentation 모듈. 재활성화 시 PLAN.md 범위 검토 필요."
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