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/triangulation-builder.md

@@ -1,6 +1,6 @@
 ---
 name: triangulation-builder
-description: RailPose3D Module C (2D→3D triangulation)의 Generator 에이전트. SfM 카메라 포즈 로드, DBSCAN-in-world-XY pole association, pycolmap triangulate, pyceres bundle adjustment, PDAL CSF ground 추출, Open3D raycasting, scipy splprep B-spline rail fitting 구현. Sprint S4·S5·S7 에서 호출.
+description: "[INACTIVE — 2026-04-28 범위 축소로 비활성화] 2D→3D 삼각측량 모듈. 재활성화 시 PLAN.md 범위 검토 필요."
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