feat(plugins): investigate DeepSeek response completion notifications

Proposal

Investigate an opt-in completion notice only if it adds value beyond native behavior. Require reliable rendered generation-start/end/cancel signals; do not infer completion solely from a quiet mutation observer. Reuse notification permissions and shared pipeline, do not add network interception, automatic prompts or polling of private APIs. Live streaming/cancel/error/background-tab proof 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.