FRE-680: Initial project scaffold with auth & API client

- Set up Go module with Cobra CLI skeleton
- Implemented login/logout/session commands with 2FA support
- Created ProtonMail API client with rate limiting
- Added config management for ~/.config/pop/
- Configured CI/CD pipeline with GitHub Actions
- Added Makefile for build/test/lint targets

Files:
- main.go, go.mod, go.sum
- cmd/root.go, cmd/auth.go
- internal/auth/session.go
- internal/config/config.go
- internal/api/client.go
- Makefile, README.md, .gitignore
- .github/workflows/ci.yml

Makefile

@@ -0,0 +1,33 @@
+.PHONY: build test clean fmt lint run
+
+BINARY_NAME := pop
+GO := go
+GOFLAGS := -v
+
+all: build
+
+build:
+	$(GO) build $(GOFLAGS) -o $(BINARY_NAME) ./main.go
+
+test:
+	$(GO) test -v ./...
+
+clean:
+	rm -f $(BINARY_NAME)
+	$(GO) clean
+
+fmt:
+	$(GO) fmt ./...
+
+lint:
+	$(GO) vet ./...
+
+run: build
+	./$(BINARY_NAME)
+
+deps:
+	$(GO) mod download
+	$(GO) mod tidy
+
+install: build
+	install -m 755 $(BINARY_NAME) $(GOPATH)/bin/