[FEATURE]: Go bindings (cpex-ffi + cgo)

[terylt]

Epic: CPEX Rust Core (#12)

Summary

Create the C FFI layer (cpex-ffi crate) and Go bindings (go/ package) as the first language binding for the Rust core. Go is prioritized for K8s deployments expressed interest and Python already has a complete implementation.

Motivation

Kagenti and other Go-based agent platforms need CPEX plugin execution with the same semantics as the Python/Rust implementations. A shared Rust core with Go bindings ensures behavioral parity without reimplementing the PluginManager in Go.

Scope

cpex-ffi crate

• #[repr(C)] result struct — continue_processing, violation_code, violation_reason, modified_payload (pointer + length)
• Opaque handle types — cpex_manager_t, managed via Arc internally
• Lifecycle — cpex_manager_new(), cpex_manager_free()
• Hook dispatch — cpex_invoke_hook() with payload as byte buffer (MessagePack default, JSON debug)
• Result access — cpex_result_blocked(), cpex_result_payload(), cpex_result_free()
• Error reporting — cpex_last_error() (thread-local)
• ABI versioning — cpex_abi_version()
• cbindgen.toml → generates include/cpex.h

Go bindings (go/)

• go.mod with module path github.com/contextforge/cpex/go, package name cpex
• cpex.go — NewManager(), InvokeHook(), Close()
• types.go — PluginResult, HookType, Extensions
• errors.go — error types bridging cpex_last_error()
• internal/ffi.go — cgo bindings to cpex.h (pkg-config + relative path fallback)
• internal/memory.go — handle lifecycle, free safety

Wire Format

• MessagePack as default (fast, compact, schema-less)
• JSON as debug option (human-readable)
• Zero-copy where possible: Go pins memory for input, Rust reads directly. C struct envelope for result scalars.

Acceptance Criteria

• go test ./... passes from the monorepo (cargo build -p cpex-ffi && cd go && go test)
• cpex.NewManager() initializes a Rust PluginManager via cgo
• cpex.InvokeHook() dispatches through the 5-phase executor
• ABI version check at init time
• Result envelope readable without deserialization (C struct scalars)
• Payload serialized only when modified