Add contribution routing foundation (GW-043)

[trippyogi]

Summary

• Add RoutingDecision contracts and a pure routeContribution policy so contribution mode stays distinct from verdict and disposition.
• Cover the routing decision table and safety mutations (no BUILD on closers, released fixes, or failed providers).
• No CLI/MCP behavior change; existing frozen verdict corpus is unchanged.

Test plan

• pnpm check (lint, typecheck, build, unit, schemas, agent-plugins)
• pnpm eval:frozen (31/31, no verdict regression)
• Bugbot on branch changes (no findings)
• Confirm later slices (GW-044+) consume RouteFacts without calling GitHub from routing