Add package doc comment and root command tests

[masnwilliams]

Summary

• Adds a Go package-level doc comment to cmd/kernel/main.go explaining what the CLI does.
• Adds tests to cmd/root_test.go covering:
  • TestRootCommandSubcommands — verifies all expected subcommands are registered on the root command.
  • TestRootCommandHelpOutput — executes --help and checks the output contains key command names.
  • TestLogLevelToPterm — table-driven test for log level string parsing, including invalid input fallback.
  • TestIsUsageError — validates detection of cobra usage error patterns.

Test plan

• Run go test ./cmd/... and confirm all new tests pass alongside existing TestIsAuthExempt.

[kernel-internal]

🔧 CI Fix Available

I've pushed a fix for the CI failure (go vet error: non-constant format string in fmt.Errorf).

The fix replaces fmt.Errorf(tt.err) with errors.New(tt.err) in cmd/root_test.go.

👉 Click here to create a PR with the fix