Extensible tag-based model classification and discovery

[sethfitz]

Replace the hardcoded namespace/ModelKind classification with tags --
string labels derived by tag providers registered through Entry Points.

system provides generic tag-based grouping without understanding what
any tag means. Any package can register tag providers that classify
models without special support in the discovery layer. Model discovery
moves from core into system in the process.

Tags serve three roles:

• CLI filtering -- select subsets of models for output and codegen
  (--tag system:feature, --tag draft)
• Classification and endorsement -- distinguish features from
  extensions, mark models as vetted or approved
• Marketplace taxonomy -- browse and classify models and extensions
  in a future extension catalog

Builds on the design from #440.

Tracked by PR #463.