chore: clean up repo — add .gitignore, README, remove tracked build artifacts and old_source

- Add .gitignore for bin/, obj/, .vs/, old_source/, IDE files
- Remove 1145 tracked files that should never have been committed
- Add README.md with project overview and build instructions
- Trim CLAUDE.md, add .claudeignore, skills, and PreToolUse hook
- old_source/ is recoverable from the initial commit (6e1b929) if needed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,27 @@
+# Build output
+bin/
+obj/
+
+# Visual Studio
+.vs/
+*.user
+*.suo
+*.userosscache
+*.sln.docstates
+
+# NuGet
+*.nupkg
+**/[Pp]ackages/*
+!**/[Pp]ackages/build/
+
+# OS
+Thumbs.db
+ehthumbs.db
+Desktop.ini
+.DS_Store
+
+# Archived .NET Framework 4.8 source (recoverable from initial commit)
+old_source/
+
+# Project-local Claude Code settings (personal permissions, not shared)
+.claude/settings.local.json