FEATURE: Add site directory list tools for categories, groups, and tag groups

[RubenOussoren]

Previously, models could only discover a site's categories and groups through MCP resources, which many clients do not surface, the listings were incomplete when category lazy-loading was enabled or groups spanned multiple pages, and there was no way to list tag groups at all.

This change adds three always-registered, read-only, parameterless tools — discourse_list_categories, discourse_list_groups, and discourse_list_tag_groups — that return complete, deduplicated directories with structured output (categories include parent_category_id so the hierarchy can be reconstructed at any nesting depth), and deprecates the discourse://site/categories and discourse://site/groups resources in favour of them.

[SamSaffron]

@RubenOussoren I think we probably want to focus on: #51

will make sure I add a toolset for it, but feel free to push to the PR if you prefer

[RubenOussoren]

@SamSaffron, if we can add the toolset for it to #51, I'm happy with that too. There's no rush for this one at the moment. We can close this PR in favour of #51 then.

Please let me know if you need me to do anything further with this then. Thanks you!

[SamSaffron]

I merged my PR now ... I think we have a toolset can you test?