feat: add commands to list the command tree for easier discovery

[developerkunal]

🔧 Changes

Adds a new top-level auth0 commands command that prints the entire CLI command tree along with a short description of what each command does. The goal is to make the whole command surface discoverable in a single place, so you (or an AI agent driving the CLI) can find the right command without inspecting each --help page individually.

It supports three output shapes:

• Default — a nested tree with descriptions, easy to scan visually.
• --flat — every runnable command on its own line as path — description, which is the easiest form to scan or match an intent against.
• --json — a machine-readable representation. Combined with --detailed it also includes the usage line, positional arguments, local flags (name, shorthand, usage, type, default), aliases, and whether the command requires authentication, which is enough to construct a valid invocation programmatically.

Additional behavior:

• Accepts an optional command path (for example auth0 commands apps or auth0 commands apps create) to scope the output to a single branch instead of the whole tree.
• A --depth flag limits how many levels are shown.
• The command does not require authentication, since it only introspects the local command tree.

The tree-walking logic runs only when auth0 commands is invoked, so there is no impact on the startup or runtime of any other command, and no new dependencies are introduced.

📚 References

N/A

🔬 Testing

• Added unit tests in internal/cli/commands_test.go covering tree building, depth limiting, detailed output, flat output (including the scoped case), and positional argument extraction that correctly ignores flag values.
• The full internal/cli test suite passes.
• Regenerated the docs so docs/auth0_commands.md and the docs/index.md listing stay in sync with the new command.

Manual checks:

auth0 commands
auth0 commands --flat
auth0 commands apps
auth0 commands apps show --json --detailed

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)