Condense AGENTS.md - #9814

• Closes AGENTS.md causes instruction overload and context rot #9704

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• I did not use AI to write the code of this patch.
• This PR follows our AI policy

Summary

AGENTS.md goes from 306 lines to 109. Supersedes #9703.

Merged

• The Lib/test marker rules and the test-modification rules were stated in two sections. They are now one section.
• The test commands were listed under both "Commit Hooks" and "Testing". They are now only under "Testing".
• Python code rules are folded into "Working with the Lib Directory".

Changed

• Required checks are scoped by change type in a table. Documentation-only changes need no test run. Clippy is listed with the Rust checks.
• The marker spelling is TODO: RUSTPYTHON; <reason> everywhere.
• The grep example is replaced with rg.
• The clean-build command uses find . -name '*.pyc' -delete.
• scripts/whats_left.py is noted as running under CPython 3.15 or newer.

Removed

• "Integration Between Rust and Python" and "Common Patterns" (pymodule example, add_native_module example).
• The Bad/Good code example for branch deduplication; the rule itself stays as one sentence.
• "Follow Rust best practices", the PEP 695 example, REPL and JIT run examples, cargo doc, and the docs.rs link.

Unchanged

• AI policy, git operation, and commit hook rules.
• Marker forms and the expectedFailure vs skip criteria.
• Comment rules, including the /// docstring rule.
• venvlauncher, zizmor, WASM build command.

Written by Claude Code (claude-fable-5-2) under maintainer direction.

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