[v2 schema] Decide what public Attribute Groups are and how they work in the weaver model.

Today, we have some very important parts of Semantic Conventions that we haven't figured out how to use in weaver's model going forward.

code.*, exception.*, thread.* are attributes that may be attached to any log or span. Today they are defined as raw attribute_group with no signal attached. These are already generated by OpenTelemetry instrumentation, relied on by users and too useful to ignore.

We need to sort out how we will model these going forward in V2 schema, particularly:

• What should live-check do when encountering a signal with these extra attributes?
• Will we allow these attribute groups on metric signals?
• Will we allow these attribute groups on entities? If yes Are they descriptive?
• Should emit place these attributes on all spans/events it generates?
• How would "out of the box" codegen interact with these attributes?
  • Will we allow "typesafe" Span/Event creation to allow extra attributes?
  • Will we require "allowed attribute groups" defined per-signal?
• How do we want documentation of attribute groups to work going forward? Can we just keep it as is (current plan)

Start with the V2 schema and Weaver model, then trace how raw attribute groups relate to live-check, metric and entity signals, emit, out-of-the-box codegen, and documentation. Resolve the checklist into a documented decision for public Attribute Groups and define what the resulting model should allow.