[Phase 14] Add packaging, installation, and release automation

[cosentinode]

Goal

Make bitbygit installable and releasable.

Scope

• Add release build workflow.
• Produce binaries for supported platforms.
• Document install methods.
• Add version output.
• Prepare for package manager distribution later.

Non-negotiables

• Code simplicity is a must: keep release automation understandable and maintainable.
• Security is a must: release artifacts should be produced by CI, with no secrets committed to the repo.
• Scalability is a must: release setup should support additional targets over time.

Acceptance Criteria

• CI can build release artifacts.
• Users can install or run a downloaded binary.
• Release docs explain supported platforms and prerequisites.

[cosentinode]

Deployment note from user: a gh-pages branch now exists and may be used as a deployment target when Phase 14 packaging/release/deployment work begins.