Add AGENTS.md

[dingsdax]

The repository-docs standard requires every SDK repo to have an AGENTS.md at the repository root. sentry-python currently has no AGENTS.md, meaning AI agents working in this repo have no architecture context, no testing conventions, no build instructions, and no SDK-specific standard overrides. The file that makes agentic contributions accurate and consistent.

What needs to be done

1. Create AGENTS.md at the repo root. Use agents-md skill.
2. Create CLAUDE.md as a symlink: ln -s AGENTS.md CLAUDE.md
3. Fill in all SDK-specific sections:
   • Package manager and key commands (pip/tox/make: tox -e linters, tox -e py3.12, make test)
   • Architecture overview (core SDK in sentry_sdk/, integrations in sentry_sdk/integrations/, tests in tests/)
   • Development workflow (tox environments, linting via tox -e linters, integration test matrix)
   • Testing conventions (pytest, test file naming test_*.py, tox environments per integration, tests/integrations/ layout)
   • Standards overrides (any known deviations from the SDK standards)
   • Evaluate adding a extra AGENTS.md or similar file for AI integrations.
4. Verify accuracy: have an AI tool attempt a small contribution using the file as context before merging

[linear]

PY-2121