Update GEMINI.md, rename to AGENTS.md

AGENTS.md

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+# 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).