fix: use protojson camelcase default for wf payload serialization

[sicoyle]

Summary

• Proto-typed workflow payloads (e.g. google.protobuf.Struct) now serialize to camelCase JSON — the protojson default and standard JSON wire convention
• Pure-Go payloads were already camelCase via json tags; this fixes mixed-case output when both flow through the same marshalData helper

Why

Workflow outputs were producing mixed camelCase/snake_case JSON depending on whether the field originated from a proto type or a Go struct. External consumers (MCP spec, JSON SDKs in other languages) expect camelCase consistently. Defaulting protojson to camelCase aligns proto payloads with that convention.

[Copilot]

Pull request overview

Switches protojson marshaling for workflow payloads from snake_case (UseProtoNames: true) to the default camelCase, aligning proto-typed payloads with Go-struct payloads and standard JSON conventions.

Changes:

• Drop UseProtoNames: true in task/executor.go protoMarshaler.
• Drop UseProtoNames: true in api/orchestration.go protoMarshaler.
• Update accompanying doc comments.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
task/executor.go	Use default protojson options (camelCase) for workflow payload marshaling.
api/orchestration.go	Use default protojson options (camelCase) for workflow payload marshaling.

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