Audit and decide what strategy.data.gov content to migrate

## Summary

Before any files move, make an explicit keep/archive/drop decision on the `GSA/data-strategy` repo content. This ticket gates all downstream migration work.

## Background

`strategy.data.gov` is being retired and redirected to `resources.data.gov`. The `GSA/data-strategy` repo has ~24 pages of content across several categories. Not all of it belongs on resources.data.gov.

## Proposed decisions (confirm or revise in comments)

**Migrate to resources.data.gov:**
- Principles and practices text (durable policy reference)
- 2020 Action Plan PDF (already linked from resources.data.gov)
- Proof points / case studies (if not already duplicated)

**Archive only, do not migrate:**
- Agency action progress pages (`pages/2020/`, `pages/2021/`)
- Team page, special-thanks, feedback pages, contact page
- News/blog posts (`_posts/`)

**Drop entirely:**
- `creating-a-strategy.md`, `incubator.md`, redirect stubs (`x-redirect-*.md`)

## Acceptance criteria

- [ ] Written decision on each content category posted as a comment on this issue
- [ ] PM sign-off before any migration work begins (Tickets 2-7 are blocked on this)