Two New Skills — Maintain Splash Pages, Chroma Agent Skills (Submodule)

Added the Maintain Splash Pages skill — codifying the <repo>/splash/ GitHub Pages pattern proven across three reference implementations — and pinned chroma-core/agent-skills as a submodule so agents loading the Lossless skills tree get upstream-authoritative Chroma guidance instead of paraphrased-from-training-data vector-store advice.