Support hierarchies of agents

[cboos]

Seen this morning in the CHANGELOG:

2.1.172
Sub-agents can now spawn their own sub-agents (up to 5 levels deep)

So, I'm glad we've already switched to hierarchical rendering (9f32a9c), but that won't be enough; we also need to rethink the CSS levels and how to stitch things together (finishing #174 first, of course).

Now, if sub-agents can start their own dynamic workflows, that's going to be even messier :-)

[cboos]

Hello @daaain, this one is cooking, and if it's not finished tomorrow morning, it'll have to wait until the 21st of June (unless you want to take over, of course).

That being said, with #174 and the additional fixes (notably addressing more weird DAG cases that led to a very branchy structure), this probably already deserves a 1.5.0 release if you feel like cutting one.

[cboos]

So, unfortunately, it didn't one-shot it. I'll have to continue later. Up to you whether to merge the intermediate PR #218 or wait until PR #219 is done.

[cboos]

Closed with #218 and #219.