Multi-capability workflow orchestration: pipeline execute surface for reference apps

[enricopiovesan]

Summary

Implement spec 058-workflow-pipeline-execution.

Governing spec

https://github.com/traverse-framework/traverse/blob/main/specs/058-workflow-pipeline-execution/spec.md (#559)

Depends on

• Add traverse-starter.validate and traverse-starter.summarize WASM agents for pipeline showcase #554, Add doc-approval.analyze capability contract, WASM agent, and app manifest #556, Add doc-approval.extract capability contract, WASM agent, and app manifest #538, Add doc-approval.recommend WASM agent for pipeline showcase #555 WASM agents

Definition of Done

• traverse-starter.pipeline + doc-approval.pipeline workflow JSON
• Runtime sequential execution with merged output
• Integration tests for both pipelines
• Dev sidecar execute accepts pipeline id

Unblocks

reference-apps #110, #111