v0.18.0 — exec-visualization: charts route to flint-chart

Added

• New exec-visualization skill — routes chart/graph tasks to microsoft/flint-chart (rendering PNG/SVG) via a capability ladder (flint-chart MCP tools → npx flint-chart CLI → Python matplotlib as a last resort), instead of ad-hoc Python plotting. The token is wired into Phase 4 of the command (with a byte-identical .claude mirror) so chart-producing tasks load it, and the README documents optional flint-chart-mcp setup for the Codex CLI and Claude.