feat(mcp): gittensory_explain_score_breakdown

[JSONbored]

Parent roadmap: #525
Parent phase: #544

Background

The private score preview is an opaque number; miners can't see which multiplier is holding them back.

Goal

New MCP tool gittensory_explain_score_breakdown that returns a plain-English, per-multiplier breakdown + the single highest-leverage lever to improve.

Current Behavior

buildScorePreview returns scoreEstimate multipliers + blockedBy + gateDeltas, but no human explanation tool.

Desired Behavior

Wraps an existing score preview and explains each component (densityMultiplier, credibilityMultiplier, openPrMultiplier, issueMultiplier, review penalty…) and what would change it. Pure projection over ScorePreviewResult (scoring/preview.ts).

Implementation Requirements

• Register in src/mcp/server.ts + package bin; outputSchema; private tool (login+repo scoped). No new computation.

Public/Private Output Boundaries

• Returned to the miner's own session only; surfaced bands/levers, raw private deltas stay private.

Acceptance Criteria

• Each multiplier is explained with a concrete "do X to improve" lever.

Testing Requirements

• npm run test:ci, 97%+ coverage. Snapshot/semantic tests over representative previews.