docs(agents): require running Python through Nix (#749)

## Summary

- Add rule to Nix section of AGENTS.md requiring Python to be run via
`nix run 'nixpkgs#python3' -- $@` when Nix is available, never bare
`python`/`python3`

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: cpcloud

AGENTS.md

@@ -202,6 +202,9 @@ details; do not duplicate that detail here.
 - **Use `writeShellApplication`** not `writeShellScriptBin` for Nix shell
   scripts. Use **`pkgs.python3.pkgs`** not `pkgs.python3Packages`.
 - **Nix package mappings**: `benchstat` is in `nixpkgs#goperf`.
+- **Run Python through Nix**: If Nix is available, always run Python via
+  `nix run 'nixpkgs#python3' -- <script.py> [args...]`. Never use a bare
+  `python` or `python3` command directly.
 
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