Foundation for the per-agent icon feature (#137).
- harness-base.nix: new hyperhive.icon option (nullable path to an
SVG). An agent commits an SVG into its config repo and references
it as ./icon.svg; when set it lands at /etc/hyperhive/icon.svg.
- web_ui.rs: GET /icon serves the configured SVG, falling back to the
bundled hyperhive logo when none is set — so it always returns an
image and consumers can hit it unconditionally.
Closes#139