refactor: adopt cobra's error handling and properly validate inputs

[mynk8]

Description

The codebase utilizes cobra but does not use it's error handling features like cobra's PreRunE, Args, and RunE hooks were never adopted, and defer statements were skipped on error paths and never surfaced due to os.Exit(0) (cannot repro this just a speculation).

This refactor improves CI reliability by routing command validation through Cobra's standard Args / PreRunE / RunE flow. Invalid CLI usage now consistently returns a non-zero exit code while preserving actionable usage output, which is important for GitHub Actions and other automated environments that rely on process status.

The changes fix the problems with input validation with proper integration of cobra in the codebase.

I have added small tests for sanity checks as well, would like some feedback on this PR :)

Changes:

• All commands: Run converted to RunE correctly returning error
• test: Args ExactArgs(3), PreRunE for runner and --waitFor validation
• import: Args MinimumNArgs(1)
• import-url: Args MinimumNArgs(1)
• import-dir: Args ExactArgs(1)
• login: Args ExactArgs(1), PreRunE for --sso-port range validation
• logout: Args ExactArgs(1)
• context: conditional arg validation in RunE
• start: PreRunE for --port range validation
• Root command: add SilenceErrors alongside existing SilenceUsage
• 24 os.Exit(1) calls removed, replaced by error returns
• log.Fatalf calls in start/stop replaced by fmt.Errorf returns
• Port validation tests for --port and --sso-port

Bugs

Addresses #281