Incorporate meeting feedback into tags design

Restructure around meeting outcomes: tags generalize the
ModelKind classifier from pydantic-extensions, with tag
providers as the mechanism from Phase 1 (not deferred).

- Add Purpose, Generalizing the Classifier, Privileged
  Packages, Security Roadmap, and Deferred Keywords sections
- Merge old Phase 1 (static tags) and Phase 2 (tag providers)
  into a single phase; tag providers are the only tag source
- Add privilege table for prefix reservation (overture:* owned
  by core, system:* owned by system) over flat reservation,
  with rationale: structured tags encode relationships, prevent
  multi-ecosystem collision, and avoid tag-pairing ambiguity
- Add feature provider (Feature in system) separate from
  overture provider (OvertureFeature in core)
- Move extension provider into Phase 2 (Extension Support)
- Defer package keywords to a future tag provider; drop #tag
  syntax from entry point format (names are just identifiers)
- Renumber phases: extensions become Phase 2, manifest-driven
  approval becomes Phase 3

Author: sethfitz