feat: Add Task Groups (Defer External Task Toml to Separate PR)

[garylvov]

Description

Successor PR to #5307 that only includes task groups, not yet external task tomls, to make the review a bit easier. I changed the format of the groups a bit relative to the previous PR in the manifest based on @bollwyvl 's suggestion, and I believe this will later make adding the external tomls cleaner.

This adds [tasks-groups] to the manifest, and associated CLI stuff to make filtering tasks easy. This also does a small sidequest to address #5276 .

After this one is merged, I will PR the external tasks toml functionality initially demo'd in #5307 . In the meantime I'll PR it within my fork for vis, but I still need to get around to that.

Fixes #4971 , #5276

How Has This Been Tested?

I tested the build release locally on a minimal example, and added some small Python and Rust tests.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Opus 4.5 within Claude Code 2.1.9

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py. <- I gave this a shot but am not confident I did it correctly

[garylvov]

Local sanity check:

[baszalmstra]

Ill defer the UX design to @ruben-arts but the code looks good!