Add AGENTS.md

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.