feat: render markdown in detail view descriptions

[kevglynn]

Summary

Renders issue descriptions as formatted markdown instead of plain text in the detail webview panel.

Before: Raw markdown syntax displayed as-is (code fences, bullet markers, headers with #, etc.)
After: Descriptions render with proper formatting — headings, code blocks, inline code, lists, tables, blockquotes, and links.

Implementation

File	Change
package.json	Added marked v17 as a runtime dependency
src/extension.ts	Lazy-initialized Marked instance with GFM + breaks; renderMarkdown() helper replaces escapeHtml() for descriptions; .markdown-body CSS using VS Code theme variables

Key design choices

• Server-side rendering: Markdown is converted to HTML in the extension host (Node.js), not in the webview. Works within the existing CSP.
• Lazy initialization: Avoids ESM/CJS issues at module load time. esbuild handles the conversion at bundle time.
• Graceful fallback: If marked fails to load or parse, falls back to escaped plain text — no breakage.
• Theme-aware CSS: All markdown styles use VS Code CSS variables (--vscode-textCodeBlock-background, --vscode-textLink-foreground, --vscode-panel-border, etc.) for native look in any theme.

Test Plan

• TypeScript compiles clean (tsc --noEmit)
• Biome lint passes (via pre-commit check)
• Descriptions with code blocks render with syntax formatting
• Descriptions with lists render as bullet/numbered lists
• Descriptions with links render as clickable links
• Plain text descriptions still render correctly
• Works in both dark and light themes

Closes #49

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