feat(mcp): add gittensory_explain_gate_disposition tool (deterministic dry-run gate reasons)

[JSONbored]

gittensory_predict_gate (src/mcp/server.ts:1186) returns a pass/fail verdict from the public .gittensory.yml, but there is no MCP tool that returns the STRUCTURED, per-rule disposition reasons behind that verdict (which specific gate rules would pass/block and why). Add a read-only companion tool that reuses the same deterministic predict-gate computation and returns the itemized rule list — pure reasoning surface, no merge/close DECISION logic (that stays maintainer-only).

Deliverables

• Register gittensory_explain_gate_disposition in src/mcp/server.ts reusing the same predictGate input shape (predictGateShape)
• Return the structured per-rule breakdown already computed inside predictGate rather than only the top-line verdict (refactor to expose it if needed, no behavior change to predict_gate)
• zod outputSchema for the itemized rule list (rule id, status, reason)
• Tests covering pass-all, one-blocking-rule, and multiple-blocking-rules branches to 97% patch coverage

References

• src/mcp/server.ts:1186
• src/scoring/preview.ts:670
• src/scoring/preview.ts:876

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2335