chore: import clean Agent Desk snapshot (1/7)

cmd/testdata/seeds/README.md

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+# Testdata Seeds
+
+This package is the single maintenance point for built-in testdata content.
+
+- Put language-specific demo text and seed metadata here.
+- Keep database writes, transactions, repository calls, and model audit fields in the resource init packages.
+- Add new languages through `seedlang.Language` and the `*Seeds(lang)` functions.