Docs: README + OCI artifact spec

[johnnyhuy]

Summary

• Document usage, compatibility, and OCI artifact spec.

Context

• Clear docs are needed for adoption and distribution.

Scope

• README with install methods (binary, brew, choco, winget, npm, docker).
• CLI usage examples and compatibility notes.
• OCI artifact spec and publishing guidance.

Out of scope

• Marketing site or blog content.

Acceptance criteria

• Docs cover all distribution and source formats.