docs(readme): align messaging around algorithm steering

[minsing-jin]

Summary

• reframes README and concept-facing docs from sculpting/manipulation-adjacent language to algorithm steering / directional control
• clarifies the core message: users own the algorithm engine and steer it through natural language plus explicit and implicit feedback
• adds a quick setup table that distinguishes required OpenAI/local mode from optional Supabase, delivery, and source-expansion keys

Verification

• rg -n "조작|조각|Sculpt|sculpt|manipulat|4-tier|4-Tier|Triple-Input|Radar|레이더" README.md docs/VISION_v3.md docs/README.ko.md docs/README.zh.md docs/LIBRARY_EXTRACTION.md docs/phase7_prd.md docs/phase_reports -g '*.md' -> no matches
• git diff --check -> passed

Closes #30