Document source install path

[tnez]

Context
V1 deploy means a source-install path from GitHub: a user should be able to go to GitHub or run a terminal command to install. See docs/v1-scope.md.

Acceptance criteria

• Decide the initial source-install command or GitHub-driven flow.
• README documents prerequisites, install, config path, first run, and update path.
• Install path does not depend on unpublished local state.
• hud --version or equivalent basic verification is documented if implemented.

Out of scope

• Homebrew tap unless it becomes the chosen source-install mechanism.
• Signed release artifacts.
• Multi-platform packaging beyond what V1 needs.

[tnez]

Implementation is complete in PR #11: #11. I could not close this issue from here because merging PR #11 into main was blocked pending explicit approval for the default-branch mutation. The PR is green and links this issue for automatic closure on merge.