docs: add CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+# Contributing
+
+## Development
+
+```sh
+go run cmd/goupkeep/main.go -demo  # starts with sample data
+ssh -p 23234 localhost              # connect to TUI
+```
+
+## Tests
+
+```sh
+go test ./...              # unit tests
+go test -race ./...        # race detector
+golangci-lint run ./...    # linting
+```
+
+## Pull Requests
+
+- Branch from `main`, PR back to `main`
+- Conventional Commits for messages (`feat:`, `fix:`, `chore:`)
+- Tests must pass, linter must be clean
+- One logical change per PR