Add attribution objective outcome and snapshot contracts

[haasonsaas]

Summary

• extend the attribution contract with objective outcome support
• add signal-driven snapshot APIs for objective and trace completion events
• generate updated Go, TS, and Python attribution bindings

Validation

• buf lint
• buf generate --path proto/attribution/v1/attribution.proto --timeout 10m
• go test ./... -count=1
• npm ci
• npm run check:package

[cursor]

PR Summary

Medium Risk
Expands public protobuf contracts and service RPC surface (new messages/enums and new RPCs), which may require coordinated client/server updates and careful backward-compatibility checks.

Overview
Extends the attribution.v1 contract to support objective outcome attribution by adding ObjectiveContext, objective-scoped summaries, and wiring these into AttributionQuery, AttributionReport, and AttributionReportSummary.

Adds a new signal-driven snapshot model: AttributionSnapshot, AttributionSignal (+ completion signal variants), and new service RPCs RecordSignals, GetSnapshot, and ListSnapshots with filtering by AttributionSubjectKind.

Updates generated Go and Python bindings (including Connect clients/handlers) for both AttributionService and EntityService, and adds MergeEntities/SplitEntity RPC contracts with supporting request/response types.

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