docs: add verified content strategy implementation status and doc guidance

docs/Content strategy/README.md

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+# Content Strategy Internal Docs
+
+This folder contains internal design notes, implementation plans, and historical migration artifacts.
+
+## Source of truth for current status
+
+Use docs-site first for externally consumed, up-to-date implementation status:
+- `docs-site/docs/features/content-strategy/implementation-status.md`
+
+## Guidance
+
+- Treat roadmap/phase files here as planning or historical context unless explicitly re-verified.
+- Before executing work from a plan in this folder, verify against current backend modules under:
+  - `backend/api/content_planning/services/content_strategy/`