test: add automated CLI test suite

[jacobbednarz]

Summary

• Add unit tests for determineAuthType and environ type
• Add integration test infrastructure: TestMain builds the binary once, helpers isolate each test with temp XDG dirs and a file-backend keyring (no OS keyring required)
• Add integration tests covering version, list (empty, api_key, api_token, multiple profiles), exec (error paths + happy path credential injection for both auth types), and add (arg validation)
• Fix main.go to propagate cmd.Execute() error so cobra arg errors correctly return non-zero exit codes
• Add CF_VAULT_BACKEND env var to openKeyring() for explicit backend selection in CI/tests

Test Plan

• go test -v -race ./... — all 23 tests pass
• gofmt -d ./cmd/ — no output
• go vet ./... — no output