Handle CLI argument parsing failures as exit code 1

[Code0x58]

Motivation

• The default flag package emits its own error and exits with code 2 on parse failures, which prevents the program from controlling exit semantics and conflicts with the desired behavior of using exit code 1 for invalid CLI usage or hard failures.

Description

• Replace the package-global flag usage with a dedicated flag.NewFlagSet(..., flag.ContinueOnError) so parse errors can be handled by our code.
• Suppress the flag package automatic output with flags.SetOutput(io.Discard) and explicitly print the parse error and call os.Exit(1) on parse failure.
• Update all flag accesses from package-global helpers to the local flags instance (Usage, NArg, Args) to keep behavior consistent.
• Update README.md to document that bad CLI args also return exit code 1.

Testing

• Ran go test ./... which succeeded (no test files present).
• Built the binary with go build -o /tmp/gitlab-ci-validate . which succeeded.
• Executed /tmp/gitlab-ci-validate --not-a-real-flag and confirmed it returns EXIT:1 and prints usage/error as expected.

---

Codex Task

[Code0x58]

Addresses #2