feat(research): smart completion detection via stop button

[intellegix]

Summary

• Replace fixed 240s/360s dom_min_elapsed guards with stop button cycle detection (appear→disappear + 3s debounce) as primary signal for research/labs modes
• Add MutationObserver injection for content stability confirmation (5s zero mutations)
• Multi-signal orchestrator: stop button (primary) → mutation/text stability (confirming) → CSS fallback (reduced guards)
• Error state detection after stop button disappears ("Something went wrong", "Rate limit", etc.)
• Reduce fallback guards: 120s/150s stability, 120s/180s DOM min-elapsed (down from 240s/300s/240s/360s)
• Vision path deprecated for research/labs completion detection (unchanged for council mode)

Expected ~65% reduction in research query time (360s → ~130s for a 2-min query).

Test plan

• Run research_query and verify completion detected within ~15s of actual generation finishing
• Verify logs show: "Stop button appeared" → "Stop button disappeared" → "Mutation stability confirmed"
• Verify no false positives (text not truncated mid-sentence)
• Verify fallback works when stop button never appears (set poll to 999999ms)
• Verify council mode still uses vision path (unchanged)

🤖 Generated with Claude Code