chore: cleanup v0.20 — remove .ai-team, diagnostic.csx, planning-docs, and old dev-docs artifacts

Preparing repository for v0.20 freeze and BlazorShift fork:
- Delete .ai-team (replaced by .squad/)
- Remove diagnostic.csx (one-off dev script)
- Remove planning-docs (old milestone tracking)
- Slim dev-docs/ by removing old PRDs, proposals, research notes
- Keep architecture/, audits/, component-specs/, migration-tests/, reports/
  (these prove component capabilities and provide baseline for BlazorShift)

This cleanup reduces repository noise and makes v0.20 a clean baseline
for the commercial BlazorShift fork.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: csharpfritz