Create release infra for a prebuilt-binary install golden path

[jmqd]

We now have README install instructions, but the current non-Nix path still requires a Rust/Cargo toolchain and a source checkout:

git clone https://github.com/trueflow-dev/trueflow.git
cd trueflow
cargo install --path trueflow --locked

That is useful for developers, but it is not a good end-user install path.

We should add release infrastructure so there is a prebuilt-binary golden path with no Cargo or other dev tooling required.

Desired outcome

• Publish versioned GitHub Releases with prebuilt trueflow binaries.
• Support an initial platform matrix, at minimum for the platforms we are willing to document publicly.
• Attach checksums and any other integrity metadata we decide to support.
• Update the README so the recommended install path is the prebuilt-binary flow.
• Keep Cargo and Nix as secondary install options for developers and power users.

Open questions

• What platform and architecture matrix do we want first, for example macOS arm64/x86_64 and Linux x86_64/arm64?
• Do we want to start with direct GitHub Release assets only, or also add something like Homebrew later?
• Should publishing be tag-driven from GitHub Actions, or managed through some other release mechanism?

Acceptance criteria

• A tagged release automatically produces downloadable trueflow binaries.
• A user can install and run trueflow without Rust or Cargo installed.
• The README documents the prebuilt-binary install path as the golden path.

[jmqd]

Triage: partially addressed by f9780cc and d16bbe6 (local packaging, smoke tests, installer/download plumbing), but there is still no tagged GitHub Release automation producing attached binaries/checksums. Keeping open.