feat(plugins): copy DeepSeek tables as Markdown and spreadsheet text

Proposal

Add an opt-in table-copy plugin using a shared first-party primitive. Copy Markdown or TSV from a specific rendered table; preserve cell boundaries and blank cells, reject unsupported merged/nested tables clearly, and neutralize spreadsheet formula prefixes. Explicit click only, no network requests, no automatic clipboard writes. Two-site fixtures, dynamic DOM cleanup, clipboard-failure feedback, and real DeepSeek light/dark evidence are required.

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.