feat(plugins): copy selected DeepSeek question and answer turns

Proposal

Investigate selected-turn Markdown copying, not a full export suite. Must preserve speaker labels, code fences and selected order; refuse unavailable virtualized selected content rather than silently omit it. No API scraping, automatic clipboard writes or private conversation uploads. Confirm live message pairing, stable identities and completeness before implementation.

Contribution workflow

The contributor requests implementation where feasible. This issue records scope before code. Maintainer confirmation of new primitive contracts is still needed; no maintainer approval is implied by opening this issue. New PRs will be submitted only after applicable checks pass, with real-browser gaps resolved rather than labelled as complete. Related DeepSeek integration: #960 and #995.