Clean up repo: prune non-dev assets, redirect build output, add .gitignore

- Remove old/unneeded material: mockups/, docs/Image (ChatGPT imgs·GIFs),
  character art docs (IMAGE_GEN_*, *.md.old, BRAND_CHARACTERS, CHARACTER_PROFILES),
  asset backups (Characters_OriginalBackup, SourceCandidates),
  unused puppet variants (LeeSoriExtended/Regenerated/Upper/V2/V3), tools/ art scripts
- Keep active assets only: LeeSori + LeeSoriDance puppets, Live2DPreview, Live2DHost, mascot PNGs
- Fix dangling references in kept docs (README, CLAUDE, ARCHITECTURE, TYPOGRAPHY, CHARACTER_UI_INTEGRATION)
- Refresh HANDOFF.md to live state (LeeSoriDance puppet, D:\Project paths)
- Redirect all build output to Build/ via Directory.Build.props (bin+obj)
- Add .gitignore and untrack previously committed bin/obj artifacts

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Directory.Build.props

@@ -0,0 +1,15 @@
+<Project>
+
+  <!--
+    모든 프로젝트(App/Core/Tests)의 빌드 산출물(bin)과 중간 산출물(obj)을
+    저장소 루트의 Build\ 아래로 모은다.  =>  D:\Project\Dansori_EQ\Build\
+      bin: Build\bin\<프로젝트명>\<구성>\<TFM>\   (예: Build\bin\DansoriEQ.App\Debug\net8.0-windows\)
+      obj: Build\obj\<프로젝트명>\<구성>\<TFM>\
+    Build\ 폴더는 .gitignore로 git에서 분리한다.
+  -->
+  <PropertyGroup>
+    <BaseOutputPath>$(MSBuildThisFileDirectory)Build\bin\$(MSBuildProjectName)\</BaseOutputPath>
+    <BaseIntermediateOutputPath>$(MSBuildThisFileDirectory)Build\obj\$(MSBuildProjectName)\</BaseIntermediateOutputPath>
+  </PropertyGroup>
+
+</Project>