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

stories/01-buy-can.md

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+# Buy Can
+
+## User Story
+
+As a thirsty bastard I want to buy a can of soda so that I can fix my thirst
+
+## Acceptance Criteria
+
+- Delivers can of choice
+- Costs money

stories/02-change.md

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+# Change
+
+## User Story
+
+As a Scrooge McDuck I want my change to be able to keep swimming
+
+## Acceptance Criteria
+
+- Returns change on request

stories/03-stock.md

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+# Stock
+
+## User Story
+
+As a vending machine owner, I want to refill my machine with cans, so that I can keep on selling drinks
+
+## Acceptance Criteria
+
+- Stock is limited
+- When stock runs out, money is returned