Graph query depth: PageRank, temporal decay, contradiction detection

[kiyeonjeon21]

src/graph/query.ts has BFS but not much beyond. Proposed additions:

• PageRank entity importance (disambiguation tie-breaker)
• Temporal decay — older edges weight down by default; --as-of <date> time-travels
• Confidence propagation — chain confidence = product of edges, not just path length
• Contradiction surfacing — why output flags contradicts edges prominently

Pure functions over SignalGraph. No I/O. Easy to unit test.

Could be split into sub-issues if someone picks one up.