Add Pipelines for cross-platform verification (Linux, Windows, macOS)

[rmems]

Context
We currently only have GitHub Actions on Ubuntu. To verify true portability of the brainstem-daemon library (especially the planned stub paths after #11 and #16), we need official CI on Windows and macOS.

Scope

• Define pipelines that run on:
  • Linux (Ubuntu)
  • Windows (latest)
  • macOS (latest)
• For each platform, run at minimum:
  • cargo fmt --check
  • cargo clippy
  • cargo build
  • cargo test
• Initially focus on the no-default-features / stub-backend path (once decoupling lands) so that we are not blocked by libzmq or git deps on every platform.
• Optionally matrix over features when the optional features are implemented.
• Cache Rust toolchain and target directories appropriately.

Labels
ci, , portability, testing

Acceptance Criteria

• Pipeline runs successfully on Linux, Windows, and macOS agents (at least the minimal/stub configuration).
• The pipeline is documented (short section in README or docs/ci.md).

This gives us an independent signal that the core library is portable.