fix(main): surface invalid-subcommand hint in agent --help - #735

In agent mode, `--help` is intercepted before clap to emit a JSON
schema. The intercept resolved the requested subcommand and fell
through a catch-all to the generic root schema (exit 0) whenever it
didn't resolve — swallowing clap's "did you mean" suggestion. So
`pup monitor list --help --agent` was less helpful than the same
command without `--help`.

Only emit a schema when the request resolves to a real command (or
no subcommand was given); for an unknown subcommand, fall through to
clap so it reports the typo with a suggestion.

- Make `find_subcommand` alias-aware so valid aliases (e.g. `audit`)
  still return the scoped JSON schema instead of clap text help.
- Extract `top_level_subcommand` to skip values of value-taking global
  flags (`--org`, `-o/--output`, `--jq`), so `--org x monitors --help`
  scopes to `monitors`, not the flag value.
- Add unit tests for subcommand resolution, alias handling, and
  top-level extraction.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>