Feature: Track opener tabs safely

[Millbot-AI]

Goal

Use Chromium opener information to identify child tabs without building an unnecessary persistent tab-history system.

Acceptance criteria

• Read and validate openerTabId in the background context.
• Confirm that the opener still exists and is unambiguous.
• Support nested opener relationships such as A → B → C.
• Treat missing, closed, or uncertain openers as a safe no-op.
• Handle moved, pinned, and grouped tabs conservatively.
• Store no browsing history beyond the minimum runtime state.