docs(release): document Nix support and pin v0.1.3

[jmgilman]

Summary

• add a locked consumer flake exposing release-cli as a package, app, and development-shell dependency on four systems
• document direct nix run, Nix profile, and project-flake consumption plus the source-build trust boundary
• advance all workflow, signer, example, mise, and contract pins to the verified v0.1.3 release revision

Verification

• public v0.1.3 release: 26 assets; all four production jobs passed
• nix run github:meigma/release/v0.1.3#release-cli -- version --json
• isolated nix profile add and installed command execution
• repository and example nix flake check --all-systems --no-build
• repository and example nix flake check
• example nix run and nix develop command execution
• local-link validation across 14 Markdown files
• mise exec -- moon run check
• git diff --check