feat(code-demo): interactive code plugin demo — Hexagon API Gateway

Adds artifacts/code-demo.html: a full interactive demo for the muse code semantic porcelain, structured to mirror the MIDI demo in quality and depth.

The scenario — 4 AI agents build a Python API gateway in parallel: - D3 commit DAG showing 3 concurrent branches merging with 0 conflicts - Force-positioned symbol graph (28 symbols, 8 files, typed by kind + agent) - Three graph modes: Graph · Dead Code (dead symbols glow) · Impact (blast radius) - 21 code dimensions heatmap across all 6 commits - Per-commit analyze panel with pre-rendered muse code command output - Symbol info panel: kind, file, agent, born SHA, callers, callees - Agent activity panel with per-agent commit and symbol ownership - 14 CLI command cards with syntax-highlighted example output - Nav cross-links between MIDI Demo ↔ Code Demo ↔ index.html