docs: document protobuf/protoc prerequisite for source builds

[ebbsanchez]

Environment

• OS: macOS 27.0
• codeseek version: current main at audit time (0.1.26 in package.json / rust-core/Cargo.toml)
• Install method: from source
• Rust: rustc 1.96.1
• Node/npm: Node v26.4.0, npm 11.17.0

Steps to reproduce

git clone --depth 1 https://github.com/CodeBendKit/codeseek.git
cd codeseek
npm install
npm run build
cd rust-core
cargo build

Expected behavior

The from-source development path in CONTRIBUTING.md / README either builds cleanly on a fresh macOS checkout, or documents that protobuf/protoc is a required system dependency before running cargo build / ./build.sh --release.

Actual behavior

npm install and npm run build pass, but cargo build fails while compiling lance-file because protoc is not installed.

The release workflow already installs protobuf on macOS before building, so this looks like a documentation/setup prerequisite rather than an application bug.

Logs / output

error: failed to run custom build command for `lance-file v0.10.6`

Caused by:
  process didn't exit successfully: `.../target/debug/build/lance-file-.../build-script-build` (exit status: 1)
  --- stdout
  cargo:rerun-if-changed=protos

  --- stderr
  Error: Custom { kind: NotFound, error: "Could not find `protoc`. If `protoc` is installed, try setting the `PROTOC` environment variable to the path of the `protoc` binary. To install it on macOS, run `brew install protobuf`. It is also available at https://github.com/protocolbuffers/protobuf/releases  For more information: https://docs.rs/prost-build/#sourcing-protoc" }

Local confirmation:

protoc --version
# zsh: command not found: protoc

A small docs fix could add brew install protobuf for macOS and the Linux equivalent to the development setup section.

[iohub]

Thank you for your feedback. The documentation will be updated shortly.