disable all CLI tools by default, as well as get_active_location (#382)

Closes https://github.com/dart-lang/ai/issues/372

- Adds a setting for tools to control whether they are enabled by default or not.
- No longer default to `all` category being enabled, instead use the tool default if we get no other setting. Allows enabling all tools with `--enable all` regardless of defaults.
- Updates readme script to add the categories and default enabled status to the table for tools.
- Also deletes the old screenshot tool which has been disabled for quite some time, and had no way of being enabled. This didn't fit in well with the new model and it was just tech debt anyways.