docs: replace Copilot instructions with agent guidelines

Removes GitHub Copilot-specific instructions and replaces with generic
agent documentation for better AI assistant compatibility

- Reorganizes content into setup, build, and development sections
- Adds explicit PR/commit guidelines and security considerations
- Maintains all technical information while improving accessibility
- Removes tool-specific references for broader AI assistant usage

Author: flexiondotorg

.github/copilot-instructions.md

@@ -0,0 +1,65 @@
+# AGENTS.md
+
+## Setup commands
+
+- Enter development shell: `nix develop` (or let direnv activate automatically)
+- Download FFmpeg libraries: `go run ./cmd/download-lib`
+- Initialise submodules: `git submodule update --init --recursive`
+
+## Build and test commands
+
+- **Full build:** `just build` — builds FFmpeg from source, regenerates bindings, compiles all
+- **Build FFmpeg only:** `just build-static ffmpeg --clean`
+- **Build static libraries:** `just build-static` (uses current GOOS/GOARCH)
+- **Regenerate bindings:** `just generate` or `go run ./internal/generator`
+- **Build examples:** `just build-examples`
+- **Run tests:** `just test`
+- **Download libraries:** `go run ./cmd/download-lib`
+
+## Code style
+
+- **Auto-generated files:** Never edit `*.gen.go` files (constants, enums, structs, functions, callbacks) — regenerate with `just generate`
+- **C string handling:** Use `CStr` type with `.Free()` for cleanup
+- **Error handling:** Wrap FFmpeg return codes with `WrapErr()` function
+- **Stream processing:** Check `AVErrorEOF` and `EAgain` in processing loops
+- **Type naming:** All FFmpeg types prefixed with `AV*` (e.g., `AVCodecContext`, `AVFrame`)
+- **CGO required:** All builds need `CGO_ENABLED=1`
+
+## Testing instructions
+
+- Run `just test` before committing
+- Tests require downloaded libraries (`go run ./cmd/download-lib` first)
+- See `ffmpeg_test.go` for version validation patterns
+
+## PR/commit guidelines
+
+- **Submodule workflow:** Configure git for fast-forward pulls only: `git config pull.ff only && git config submodule.recurse true`
+- **Version schemes:** Two distinct versions — library releases (`lib-X.Y.Z.N`) and module releases (`v-X.Y.Z.N`)
+- **Release tags:** Library releases use `lib-` prefix; Go module releases use `v` prefix
+
+## Development workflow
+
+- **Never run `go build` directly** — always use `just build` for proper CGO flags and build sequencing
+- **Cross-compilation:** Set `GOOS` and `GOARCH` before downloading: `GOOS=darwin GOARCH=arm64 go run ./cmd/download-lib`
+- **Platform-specific builds:** Justfile auto-detects current platform, outputs to `lib/<os>_<arch>/`
+- **Binding regeneration:** Required after FFmpeg header changes — run `just generate`
+
+## Key architecture
+
+- **Core:** `ffmpeg.go` contains CGO directives, platform linker flags, helper types
+- **Generated bindings:** `*.gen.go` files in root directory
+- **Headers:** `include/` contains FFmpeg C headers
+- **Libraries:** `lib/<os>_<arch>/` contains platform-specific static libraries (gitignored)
+- **Builder:** `internal/builder/` compiles FFmpeg + 20 dependencies from source
+- **Generator:** `internal/generator/` parses headers using libclang, outputs Go bindings
+- **Downloader:** `cmd/download-lib/` fetches pre-built libraries from GitHub Releases
+
+## Hardware acceleration
+
+Supported: NVENC/NVDEC (Linux), QuickSync (Linux), VideoToolbox (macOS), Vulkan Video (cross-platform). See `README.md` and `docs/CODECS.md` for codec matrix.
+
+## Security considerations
+
+- **GPL licensing:** Combined work inherits GPL requirements from FFmpeg/x264/x265
+- **Static libraries gitignored:** Only submodule reference committed, not ~100MB binaries
+- **Library distribution:** Use GitHub Releases for pre-built binaries, not git