feat: 'Why this answer' trace — trust signal in Ask page

[rajnavakoti]

Description

Show a "grounded in" section below every Ask answer displaying which entities contributed to the answer, confidence level, and any detected gaps. This is a trust signal — it tells users the answer is grounded in the KB, not hallucinated. Always visible, not collapsed.

Acceptance Criteria

• Below every answer: top 3-5 entity pills (clickable → Explorer) sorted by contribution
• Confidence summary line: "High confidence (5 entities, 22 citations)" or "Partial — 2 gaps detected"
• Gap list in plain language: "The KB doesn't document X"
• Score badge: CLEAN / INCOMPLETE / MISSING with layer color treatment
• Uses existing retrieval trace data (no additional API calls)
• Renders inline, not in a collapsible panel

Out of Scope

• Dev trace panel with full pipeline stages (P2)
• Retrieval analytics across sessions