feat(agent): local gittensor-root scorer MCP wrapper

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Parent epic: #770 (Phase 2).

Goal

MCP tool to invoke the local gittensor-root scorer and feed token scores into the score preview (closes the 'miner runs scorer manually' gap).

Deliverables

• MCP tool run_local_scorer(changed_files, validation_output) → token scores
• Wire output into local-branch score preview (external_command mode)

Context

src/signals/local-branch.ts (scorer modes), local-scorer-diagnostics.ts.

Milestone: M4.

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Build-vs-consume note (from #813 / upstream #1460): if gittensor ships gitt miner advisor / gitt miner scan (runs the official validator scoring pipeline locally), this wrapper should consume that output rather than re-implement/mirror the pipeline. That eliminates the constant-drift class (#806/#807) and repositions gittensory as the quality/orchestration layer on top of the official scoring primitive. Design this tool to abstract the scorer source: local gittensor-root today, gitt miner advisor if/when it ships.

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Softener (per owner): consuming gitt miner advisor is an optional accuracy source if it ships — not a strategic necessity and not a competitive play. Primary path remains our own local gittensor-root wrapper + accurate sync (#814). Keep the scorer source abstracted so a future gitt-based source can slot in, but don't gate this issue on upstream.