docs: fix AGENTS.md's broken DEVELOPMENT.md references (#8426)

DEVELOPMENT.md was renamed to CONTRIBUTING.md in #7914, but AGENTS.md
wasn't updated. Repoint the two links with a clear target (Linux/macOS
testing section, development guide). Drop the "CPython Version Upgrade
Checklist" reference under venvlauncher, which never pointed to real
content in any revision of DEVELOPMENT.md.

Related to #8356.

Assisted-by: Claude Code:claude-sonnet-5

AGENTS.md

@@ -250,12 +250,10 @@ cargo run --features jit
 
 ### Linux Build and Debug on macOS
 
-See the "Testing on Linux from macOS" section in [DEVELOPMENT.md](DEVELOPMENT.md#testing-on-linux-from-macos).
+See the "Testing on Linux from macOS" section in [CONTRIBUTING.md](CONTRIBUTING.md#testing-on-linux-from-macos).
 
 ### Building venvlauncher (Windows)
 
-See DEVELOPMENT.md "CPython Version Upgrade Checklist" section.
-
 **IMPORTANT**: All 4 venvlauncher binaries use the same source code. Do NOT add multiple `[[bin]]` entries to Cargo.toml. Build once and copy with different names.
 
 ## Test Code Modification Rules
@@ -286,7 +284,7 @@ If you modify any file under `.github/workflows/`, the change must pass a [zizmo
 ## Documentation
 
 - Check the [architecture document](/architecture/architecture.md) for a high-level overview
-- Read the [development guide](/DEVELOPMENT.md) for detailed setup instructions
+- Read the [development guide](/CONTRIBUTING.md) for detailed setup instructions
 - Generate documentation with `cargo doc --no-deps --all`
 - Online documentation is available at [docs.rs/rustpython](https://docs.rs/rustpython/)
 - [How to update test files](https://github.com/RustPython/RustPython/wiki/How-to-update-test-files#checkout-cpython-source-code-initial-setup) — guide for syncing test cases from upstream CPython into the `Lib/` directory