v0.6.0 — fix broken figure links, add /ds-brief for non-technical audiences

Fixed

• Every figure link in every benchmark's 07-evaluate.md/08-explain.md/02-explore.md was broken — written as ../.last-ds-mile/figures/... from a file already inside .last-ds-mile/stages/, adding a nonexistent extra .last-ds-mile/ segment (stages/ and figures/ are siblings, not nested). This is also why no chart ever rendered inline on GitHub — the figures were generated correctly, only the reference to them was wrong. Fixed across all 14 image references in all three benchmarks; verified every relative link in benchmarks/ and showcase/ now resolves to a real file.
• Swept all three benchmarks for other staleness (artifact filenames, cross-stage number consistency, stray /ds-iterate references) — found nothing else wrong.

Added

• ds-brief skill and /ds-brief command — translates /ds-report's technical narrative into a one-page, jargon-free brief for non-technical stakeholders (executives, a board, a frontline team lead). No metric names or statistical terms permitted; every claim must trace back to /ds-report rather than introducing new analysis. Demonstrated with real examples for all three benchmarks (09b-brief.md), each verified jargon-free and under 350 words.
• CONTRIBUTING.md, SECURITY.md, CLAUDE.md — contribution workflow, private vulnerability reporting, and repo-specific instructions for Claude Code sessions working in this codebase.

Full Changelog: https://github.com/stamkavid/last-ds-mile/blob/main/CHANGELOG.md