docs: migrate dotfiles agent instructions

src/dotfiles/AGENTS.md

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+# Dotfiles instructions
+
+## Gotchas
+
+- The registry collector reads only top-level host `config.registry` declarations.
+  Registry declarations inside guest interiors live under `config.containers.<guest>.config.registry`.
+  The collected registry does not include those guest-interior declarations.
+  Publish guest values from host-side code.
+  See http://git.alexion.dev/alexion/dotfiles/issues/65.