feat: tab-complete --output-fields and enum flag values

The dynamic completer already does commands, flags, and `--use-context` values.

Extend `__complete` so `--output-fields` offers top-level keys from the command's output schema when we have one, and enum flags offer their schema enum values.

Skip if the command has no schema. Do not call the cluster to complete index names in MVP.

Acceptance: completing `--output-fields` on `elastic status` offers `services` (or whatever the schema says). Test with the `__complete` callback.