feat: add release-infographic skill

[nujovich]

Summary

Adds a project-scoped Claude Code skill, release-infographic, that generates a mint-branded PNG summarizing what ships in a release, sourced from CHANGELOG.md.

It is a skill invoked by Claude (applies editorial judgment: highlights, copy, layout) — not an automatic hook on npm run release. Rendering is deterministic HTML→PNG via a headless browser, so brand tokens and technical identifiers (mint-ds lint, gap-rule-color, …) come out exact — unlike AI-image infographics, which garble technical text.

What's included

• .claude/skills/release-infographic/SKILL.md — the editorial workflow.
• template.html — mint-branded shell (tokens from app/globals.css inline) with {{placeholders}} and three layout presets (bento, linear, dashboard).
• render.mjs — HTML→PNG via Playwright, with an injectable launch so unit tests run without a real browser.
• render.test.mjs — Vitest unit tests (fake browser; no Chromium in vitest run).
• EXTEND.md — pinned preferences (layout, language, tagline, width).
• release-assets/ — generated PNGs + HTML records, versioned; auto-excluded from the npm tarball (the files allowlist).

Supporting changes

• package.json — adds playwright as a devDependency (renderer engine).
• vitest.config.ts — excludes .claude/worktrees/** from test discovery (fixes pre-existing worktree pollution).

Test Plan

• npm test — 520 tests pass, including 3 new renderer tests
• npm run check:pack — passes; release-assets/ confirmed absent from the npm tarball
• End-to-end: rendered real CHANGELOG sections (Unreleased and 0.2.0) to valid 1200px-wide PNGs, no leftover placeholders, brand identity and code chips correct
• Width parameter verified configurable (design reflows to fill the viewport width)
• Cleanup-on-error verified (browser closes even when screenshot throws)