Update GEMINI.md, rename to AGENTS.md

This change improves the AGENTS.md (previously GEMINI.md) file to 
provide better context for AI agents working on the dcsctp library. This 
ensures generated code adheres to the library's design principles and 
reduces the likelihood of agents suggesting incompatible patterns.

It's reduced in size, and only containing the important instructions.

Author: boivie

AGENTS.md

@@ -0,0 +1,38 @@
+# AGENTS.md
+
+## Project Overview
+
+`dcsctp` is a Rust implementation of the Stream Control Transmission Protocol
+(SCTP, [RFC 9260](https://www.rfc-editor.org/rfc/rfc9260.txt)) designed for
+WebRTC Data Channels ([RFC 8831](https://www.rfc-editor.org/rfc/rfc8831.txt)).
+It is a user-space library intended to be embedded in larger systems (like
+WebRTC implementations), not a standalone server or in an operating system
+kernel.
+
+## Architecture
+
+- **Core Design**: The library is **single-threaded** and **event-driven**.
+  It does not perform I/O directly. The consumer drives the loop by feeding
+  packets/timer events and handling outgoing commands.
+- **Entry Point**: The primary public interface is the `DcSctpSocket` trait
+  (`src/api/mod.rs`). The implementation is `Socket` (`src/socket/mod.rs`).
+- **Internal**:
+  - `src/socket/`: State machine, connection establishment/teardown.
+  - `src/rx/`: Receiver logic (congestion control, reassembly).
+  - `src/tx/`: Transmitter logic (congestion control, retransmission).
+  - `src/timer/`: Timer abstractions (does not use system timers).
+- **C++ Bindings**: `src/ffi.rs` uses `cxx` to expose the Rust API to C++.
+  Changes to `src/api/` must often be reflected in `src/ffi.rs`.
+
+## Development Workflow
+
+- **Build**: `cargo build`
+- **Test**: `cargo test` (Runs extensive unit and integration tests).
+- **Lint**: `cargo clippy --all-features --all-targets -- -D warnings`
+- **Format**: `cargo +nightly fmt --all -- --check`
+
+## Contribution Rules
+
+- **Changelog**: All functional changes must be documented in `CHANGELOG.md`
+  under the *Unreleased* section (categories: Added, Changed, Deprecated,
+  Removed, Fixed, Security).