feat: add ParseAndRun convenience function

[mfridman]

This PR adds a ParseAndRun function that combines Parse and Run into a single call, handling flag.ErrHelp internally by printing usage to stdout and returning nil. This eliminates the boilerplate every caller had to write to check for help flags and print usage manually.

The caller's main() goes from ~12 lines down to ~4:

if err := cli.ParseAndRun(ctx, root, os.Args[1:], nil); err != nil {
    fmt.Fprintf(os.Stderr, "error: %v\n", err)
    os.Exit(1)
}

Parse and Run remain available for users who need two-phase initialization (e.g., setting up resources based on parsed flags before execution). Both examples are updated to use ParseAndRun.