[codi-rs][P0] Add CI matrix for Linux/macOS/Windows (build/test/fmt/clippy)

[laynepenney]

Parent tracker: #294

Problem

codi-rs currently has no GitHub Actions workflows, so there is no automated cross-platform gate for production readiness.

Scope

• Add CI workflows for Linux/macOS/Windows
• Include jobs for:
  • cargo build --release
  • cargo test
  • cargo fmt --check
  • agreed clippy policy command
• Set branch protection expectations after CI is stable

Acceptance Criteria

• CI runs automatically on PRs and main
• Matrix is green on all three OSes
• Required checks are documented

[laynepenney]

Implemented in https://github.com/laynepenney/codi-rs/pull/6\n\nAdded CI matrix workflow covering build/test on ubuntu/macos/windows, plus advisory fmt/clippy jobs while existing debt is reduced.