Update AGENTS.md for C# project setup

- Rewrite AGENTS.md from C++inary to C# (.NET 8, NUnit, dotnet test)
- Add .gitignore entries (mise.toml, .local, *.log)
- Add stories/ (buy-can, change, stock user stories)
- Add CLAUDE.md
- Remove stale .pi/skills symlink
- Keep intentional failing test in TestVendingMachine.cs

.gitignore

@@ -2,7 +2,9 @@ bin
 packages
 obj
 *.userprefs
+*.log
 .idea
 VendingMachine.sln.DotSettings.user
 donstro/remote.secrets
-
+mise.toml
+.local