fix: copy artifacts as enriched text (html + plain)

[cnewell0]

HTML and markdown artifacts now write both text/html and text/plain to the clipboard via ClipboardItem, so pasting into Docs/Outlook/Slack keeps formatting while terminals and plain-text fields still get clean source. Falls back to writeText when ClipboardItem isn't available or the rich write throws.

Description

Related Issues

Fixes #

Testing

• Manual testing
• Unit tests added/updated
• E2E tests (if applicable)

Checklist

• Read and followed CONTRIBUTING.md
• Based on latest main branch
• npm run lint passes
• npm test passes
• Unit test coverage maintained/improved
• Updated README.md (if applicable)

Contributor License Agreement

• I have signed the Contributor License Agreement

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