feat(protocol): add workflow update payload

[hishamkaram]

Summary

• Add workflow_update wire type and workflow status constants
• Add sanitized WorkflowPayload, usage, phase progress, and agent progress structs
• Cover JSON roundtrip and script/path omission invariants

Verification

• go test ./... -run TestWorkflowPayload -count=1
• go test -race -count=1 -p 4 ./...
• go vet ./...
• git diff --check

Notes

• This PR is required before agentd can pin protocol workflow symbols for standalone builds.