feat(cli): show help when no args are provided

[warnaa]

Print the root help output and exit successfully when the CLI is invoked without flags or commands. Add e2e coverage so the no-args behavior stays aligned with --help.

before

> 10x
> 

after

> 10x
10x/1.6.0

Usage:
  $ 10x <command> [options]

Commands:
  auth           Authenticate with 10xDevs via magic link
  get <ref>      Fetch and apply a lesson pack
  list [module]  Browse available modules and lessons
  doctor         Diagnose auth, API, config, and tool directory state

For more info, run any command with the `--help` flag:
  $ 10x auth --help
  $ 10x get --help
  $ 10x list --help
  $ 10x doctor --help

Options:
  --json         Output as JSON (auto-detected when piped)
  --verbose      Show detailed output on stderr
  -h, --help     Display this message
  -v, --version  Display version number

[warnaa]

Pre-existing issue is that some tests are failing😅 It felt out of scope to fix here but not sure if my setup (windows) is the problem or it's a global one.

9 tests failed:
✗ output > forces json mode when stdout is not a TTY [0.29ms]
✗ migrateArtifacts > refuses to migrate a symlinked source and records it as skipped [3.02ms]
✗ e2e: auth flow > (unnamed) [0.34ms]
✗ e2e: auth without credentials > (unnamed) [0.14ms]
✗ e2e: doctor > (unnamed) [0.22ms]
✗ e2e: get > (unnamed) [0.26ms]
✗ e2e: global flags > (unnamed) [0.15ms]
✗ e2e: list > (unnamed) [0.23ms]
✗ compiled binary > binary exists after build:binary [0.17ms]