feat(mcp): harden local scorer adapter setup

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Problem

Local score preview can be powerful, but GITTENSOR_SCORE_PREVIEW_CMD setup and failure handling need to be clearer for real MCP users.

Desired behavior

Improve the local scorer adapter so users get structured diagnostics for missing commands, timeouts, malformed output, and fallback behavior while keeping source upload unsupported.

Implementation notes

• Keep the adapter metadata-only.
• Normalize scorer output into the existing score preview shape.
• Add timeout and malformed-output handling.
• Provide a reference example or wrapper for users who have a local Gittensor scoring checkout.
• Make fallback guidance actionable from doctor, preflight, and branch analysis output.

Public/private boundaries

Local scorer output is private MCP/API context only. Do not surface score estimates in public GitHub comments.

Acceptance criteria

• Missing scorer command produces clear setup guidance.
• Timeout produces a bounded fallback result.
• Malformed JSON does not crash analysis.
• Metadata-only fallback still works.
• Source contents are never uploaded.

Tests

• Scorer success.
• Missing command.
• Timeout.
• Malformed JSON.
• Non-zero exit.
• Source-upload guard.