v0.7.1 — aicertify demo + Markdown renderer fix

Added

• aicertify demo subcommand — a self-contained, no-config demo entry point. Loads a bundled sample contract (aicertify/_demo/sample_contract.json), runs an OPA evaluation against the EU AI Act policy set, and writes aicertify_demo_report.md to the current directory. Requires only the opa binary on PATH (no API keys, no contract file). The CLI now also detects a missing opa binary and prints a one-line, platform-aware install command instead of stack-tracing.
• aicertify evaluate subcommand — the previous flat CLI behaviour, now under an explicit subcommand. The pre-0.7.1 invocation aicertify --contract X --policy Y … is still accepted (transparently routed to evaluate).
• Updated README Quick Start — collapses to three commands: pip install aicertify, curl … opa, aicertify demo. Honest first-install timing called out (~3–5 min for deps + the one-time OPA install).

Changed

• Visual refresh — all README diagrams replaced with hand-authored, theme-aware SVGs in diagrams/. Each diagram now ships as a paired _light.svg + _dark.svg and is embedded via <picture> so GitHub light- and dark-theme readers each see the variant that matches their canvas. The top-of-README logo is replaced with a hero banner SVG. The previous matplotlib generator (diagrams/generate_diagrams.py) and 5 baked-in PNG diagrams have been removed in favour of the hand-authored SVG system documented in diagrams/STYLE.md.
• OG / social card — added diagrams/og_card.png (1200×630) for GitHub Settings → Social preview, plus its _light / _dark SVG sources.
• AGENTS.md — added a "Diagrams and visual assets" section pointing future agents at the new style system.