Add AGENTS.md and CLAUDE.md

AGENTS.md

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+* Brevity is good.
+* Assume that the maintainers and readers of the code you write are Go experts:
+   * Don't use comments to explain the obvious.
+   * Use self-explanatory variable and function names.
+   * Use short variable names when the context is clear.
+* Never export symbols that's not needed outside of the package.
+* Avoid global state at (almost) all cost.
+* This is a project with a long history; assume that a similiar problem has been solved before, look hard for helper functions before creating new ones.
+* Use `go test ./somepackage/...` when iterating.
+* Use `mage check` when you're done.

CLAUDE.md

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+@AGENTS.md