Add unit tests for ui/markdown.js and an npm test script

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What is this about?

Problem

Cipher has no automated tests. ui/markdown.js is pure functions — perfect first test target. package.json has no test script.

Where to look

• ui/markdown.js — renderAssistantMarkdown, escapeHtml
• New file: scripts/test-markdown.mjs
• package.json — add "test": "node scripts/test-markdown.mjs"

Acceptance criteria

• Tests cover headings, lists, inline code, fenced code blocks, HTML escaping
• npm test passes locally
• CONTRIBUTING.md mentions how to run tests

Skills needed

Node.js only — no Electron window needed. Great first open-source test PR.