Add AGENTS.md

[dingsdax]

The repository-docs standard requires every SDK repo to have an AGENTS.md at the repository root. sentry-rust 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.

Note: sentry-rust has a CLAUDE.md at the root but no AGENTS.md. Per the standard, AGENTS.md is the required canonical file; CLAUDE.md should be a symlink to it.

What needs to be done

1. Review the existing CLAUDE.md content (via agents-md skill).
2. Rename/move its content into AGENTS.md (or create AGENTS.md with the content)
3. Replace CLAUDE.md with a symlink: ln -s AGENTS.md CLAUDE.md
4. Expand content to cover all required sections if the existing CLAUDE.md doesn't already:
   • Package manager and key commands (Cargo, rustfmt, clippy)
   • Architecture overview (workspace crates: sentry-core, sentry, sentry-actix, sentry-tracing, sentry-tower, sentry-log, etc.)
   • Development workflow (cargo build, cargo test, cargo fmt, cargo clippy)
   • Testing conventions (unit tests alongside source, integration tests, cargo test matrix)
   • Standards overrides (any known deviations from the SDK standards)
5. Verify accuracy: have an AI tool attempt a small contribution using the file as context before merging.

[linear]

RUST-167