Remove UsageFunc dispatch from DefaultUsage to fix infinite recursion

[Copilot]

DefaultUsage checks UsageFunc and calls it if set, making it impossible to call DefaultUsage from within a UsageFunc without infinite recursion. The function name implies "give me the default behavior" but instead re-dispatches to custom logic.

Changes

• usage.go: Remove the UsageFunc guard from DefaultUsage so it always produces default output
• run.go: Move UsageFunc dispatch to ParseAndRun — check UsageFunc first on help, fall back to DefaultUsage

This makes DefaultUsage composable:

cmd.UsageFunc = func(c *cli.Command) string {
    return cli.DefaultUsage(c) + "\n\nExamples:\n  myapp --verbose"
}

Previously required a workaround of nilling out UsageFunc before calling DefaultUsage.

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.