Point agents at the stdlib sync methodology.

The methodology governs how this codebase is kept in sync with CPython, but
nothing in the repository pointed agents to it.

Author: jaraco

(meta)/agents/stdlib.md

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+This project shares its codebase with the CPython standard library, where it appears as `importlib.metadata`. The two trees are kept byte-identical: changes made here are synced downstream into CPython, and changes made in CPython are ported back here.
+
+Before changing anything under `importlib_metadata/`, read the [development methodology](https://github.com/python/importlib_metadata/wiki/Development-Methodology) in its entirety. It governs how the two codebases are kept in sync, which files are excluded from the stdlib, and how to verify the trees agree.