Fix markdown rendering in task descriptions

[Thanhdn1984]

Fixes #3.

Changes:

• render safe inline markdown for task text: bold, italic, code, links
• keep text inserted via textContent/createTextNode to avoid HTML injection
• external links open safely with rel="noopener noreferrer"

Verification:

• npm test: 5/5 passing
• npm run lint: pass
• git diff --check: pass

[codebounty-dev-bot]

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