docs: document install and release workflow

[owainlewis]

Summary

• add stable install and update instructions to the README
• keep cargo run documented as the developer path
• document tagged releases, release assets, installer updates, and nightly artifacts in docs/release.md
• distinguish stable GitHub Releases from unstable Nightly workflow artifacts

Verification

• bash -n install.sh
• ./install.sh --help
• cargo run -- --version
• cargo run -- --check-update
• cargo test
• git diff --check
• subagent review found one artifact-name nit, fixed before commit

Closes #38