Add dedicated aicpp devcontainer, CI, and release packaging

[Copilot]

This addresses the missing developer environment and automation around the cpp/ implementation. It adds a ready-to-use devcontainer for aicpp bootstrapping and a dedicated GitHub Actions path for C++ build/test and release artifact packaging.

• Dev container

  • Adds a dedicated .devcontainer for the cpp/ implementation
  • Installs the required native toolchain and system deps (readline, ninja, doxygen)
  • Preconfigures CMake to bootstrap cpp/build on container creation

• aicpp CI

  • Adds a separate aicpp workflow instead of coupling C++ builds to the existing Rust CI
  • Builds and tests cpp/ on Ubuntu and macOS
  • Limits workflow token permissions to read-only for the CI path

• aicpp release packaging

  • Adds a dedicated release workflow for the C++ implementation
  • Rebuilds and tests before packaging
  • Produces install-tree archives from cmake --install and publishes them on tagged releases

• Documentation

  • Updates cpp/README.md and cpp/QUICKSTART.md to surface the devcontainer and automation entry points
  • Links the C/C++ implementation guide from the root README.md

Example of the added bootstrap flow:

{
  "name": "aichat-aicpp",
  "postCreateCommand": "cmake -S cpp -B cpp/build -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo -DBUILD_TESTS=ON -DBUILD_EXAMPLES=OFF"
}

[drzo]

cooool