feat: implement Markdown domain in GitHub API simulator

[Copilot]

Summary

POST /markdown and POST /markdown/raw previously returned random stub responses. This implements a functional Markdown-to-HTML renderer backed by a proper Context class, with deterministic output and correct response headers.

Original Prompt

Implement Markdown domain in the GitHub API simulator and create a PR

Manual acceptance tests

• POST /markdown with {"text": "# Hello"} returns 200 with Content-Type: text/html and body containing <h1>Hello</h1>
• POST /markdown with {"text": "**bold** and *italic*"} returns body with <strong>bold</strong> and <em>italic</em>
• POST /markdown with {"text": "See #42", "mode": "gfm", "context": "octocat/hello-world"} returns a link to https://github.com/octocat/hello-world/issues/42
• POST /markdown/raw with plain-text body # Raw returns 200 and body containing <h1>Raw</h1>
• Both endpoints return an X-CommonMarker-Version header
• Fenced code block containing <script> returns <script> — not a live tag

Tasks

• routes/markdown/_.context.ts — new Context class; renders headings, paragraphs, bold/italic/strikethrough, inline code, fenced code blocks (with language class), lists, blockquotes, HR, links, images, and GFM issue refs (#42 → <a href="…">); escapes HTML in code blocks
• routes/markdown.ts — delegates to context; sets Content-Type, Content-Length, X-CommonMarker-Version
• routes/markdown/raw.ts — delegates to renderRaw(); sets X-CommonMarker-Version
• routes/_.context.ts — adds MarkdownContext delegation: renderMarkdown, renderRaw, commonMarkerVersion
• test-support/create-context.ts — registers /markdown context path
• test-support/create-response.ts — shared response builder mock with arbitrary header-chain support
• test/markdown.context.test.ts — 15 unit tests for Context
• test/markdown.routes.test.ts — 5 HTTP-level route tests

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