feat: add opt-in help-all command (ref #2362)

[dpritchett]

Summary

Adds NewHelpAllCommand() — an opt-in command that prints every visible, runnable command with args and description in one pass. Same wiring pattern as completion:

rootCmd.AddCommand(cobra.NewHelpAllCommand())

• One line per command, columns aligned
• --verbose adds flag placeholders with type-aware formatting
• ~160 lines, no new dependencies

Context in #2362 (see my comment). Gist with gh output (~200 commands).

Example (patched into a local clone of gh)

$ gh help-all
    gh api <endpoint>                                            # Make an authenticated GitHub API request
    gh issue create                                              # Create a new issue
    gh pr merge [<number> | <url> | <branch>]                    # Merge a pull request
    gh repo clone <repository> [<directory>] [-- <gitflags>...]  # Clone a repository locally
    gh run watch <run-id>                                        # Watch a run until it completes
    ...

Full gist with ~200 commands

What's here

• helpall.go: factory, tree walker, formatter, renderer
• helpall_test.go: 19 tests

Roadmap

• Implementation + tests
• Proof of concept against gh CLI
• Maintainer feedback on design
• Doc page in site/content/ if accepted
• Final review
• Land

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