feat(mcp): gittensory_check_slop_risk tool + real-vs-slop rubric

[JSONbored]

Parent roadmap: #525
Parent phase: #526

Background

Miners want to know, before opening a PR, whether their work will read as slop under Gittensor's rules. We can give them that without ever uploading source: score metadata, and hand the rubric to their own local harness.

Goal

Add a new MCP tool gittensory_check_slop_risk plus fold slop into the existing local preflight path.

Current Behavior

grep check_slop_risk is empty. Preflight tools exist but do not surface a slop verdict.

Desired Behavior

• New tool registered in src/mcp/server.ts next to gittensory_preflight_local_diff, reusing the existing localBranchAnalysisShape input (no new client plumbing).
• Returns slopRisk + per-signal { reason, howToFix } + SLOP_RUBRIC_MARKDOWN so the miner's own harness self-audits the actual code.
• Fold buildSlopAssessment findings into buildLocalBranchAnalysis so preflight, explain_local_blockers, and prepare_pr_packet surface slop risk for free.

Implementation Requirements

• Depends on the slop core (roadmap(pillar 1): anti-slop contribution quality layer #526 child). No source upload — metadata only.
• howToFix strings authored to pass the public-safe filter so they can appear in the drafted PR body.

Public/Private Output Boundaries

• No reward/score/wallet/hotkey/trust language. Source contents must never be transmitted (fail closed).

Acceptance Criteria

• A slop-y local diff returns elevated/high risk with specific, actionable reasons + the rubric.
• A clean diff returns clean/low. Output is public-safe.

Testing Requirements

• npm run test:ci, 97%+ coverage. Unit tests over representative diffs; assert no source content leaves the process; forbidden-term assertions on output.