proposal: added golang bindings for CPEX core

[terylt]

Summary

Adds the CPEX Go Bindings Proposal — a design document for embedding the CPEX Rust plugin runtime in Go applications
via C FFI and MessagePack serialization.

Changes

• Golang Bindings Proposal — CPEX-centric Go bindings proposal for the CPEX repo

What the proposal covers

• Architecture: Rust plugins with a Go host. Plugins written in Rust for security guarantees and native execution.
  Go manages lifecycle and invocation. Single FFI crossing per invoke — no per-plugin serialization round-trips.
• MessagePack boundary: Payloads and extensions serialize via MessagePack. ContextTable and BackgroundTasks are
  opaque Rust handles passed by pointer — never serialized.
• Go SDK: PluginManager wrapper with NewPluginManager, Initialize, Shutdown, InvokeByName, typed
  Invoke[P], HasHooksFor, PluginCount. Clean Go API hiding all C FFI internals.
• Security guarantees: Immutable payloads, monotonic sets (add-only labels), append-only delegation chains,
  guarded fields (write tokens), capability gating enforced by Rust's type system.
• Full extension model: MetaExtension (routing), SecurityExtension (Subject, AgentIdentity, claims),
  DelegationExtension (scope-narrowing chain), HttpExtension (headers, method, path), Labels (monotonic set),
  FilteredExtensions (capability-gated view).
• CMF types: Go mirror of the Python CMF Message model — Role, Channel, ContentPart (text, image, tool_call,
  tool_result, resource, prompt, etc.), ToolCall, ToolResult, Resource, Metadata.
• Complete example: Go host loading YAML config, invoking CMF pre/post hooks with routing, threading ContextTable
  across hooks, awaiting BackgroundTasks.
• Planned features: Reverse-order response hooks, required plugins, bypass/passthrough, adapter helpers, body
  buffering budget.
• Future extensions: Go-native plugins (Option C) with optimized FFI for the no-modification fast path. WASM
  plugins with sandbox tradeoffs.
• C FFI surface: Minimal API with opaque handles for manager, context table, and background tasks.

Test plan

• Design review with team
• Validate Go SDK types against Rust core types
• Prototype C FFI for cpex_invoke round-trip
• Benchmark MessagePack encode/decode overhead for typical CMF payloads

[araujof]

Superseded by #45