v0.3.0

Public-ready polish for html-explainer.\n\n- Adds clearer public README signals, contribution guide, security policy, and CODEOWNERS.\n- Tightens example validation for dark-mode CSS and duplicate next prompts.\n- Cleans bundled examples so generated artifacts model the intended quality bar.\n- Adds worked skill examples for simple, medium, and complex intent-led HTML use cases.\n\nCI passed on main for commit 879f173.