feat(catalog): add reusable catalog - #362

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Codex PR Review

Findings

High: The PR does not compile after changing the loader signatures. LoadFromBytes now returns three values, but production code still expects two in the MCP validator, and cmd/run/workflows.go assigns reg without using it. Tests also still use the old signature. Update all call sites to handle the registry return, or assign it to _ where it is genuinely unused.
loader.go, cmd/run/workflows.go, pkg/mcp/validate_workflow.go

High: call: catalog is accepted by the schema and routed through the task factory, but NewCallCatalogTaskBuilder returns nil, nil. The enclosing DoTaskBuilder then calls builder.PostLoad() or builder.Build(), which will panic for any workflow using the new task type instead of failing explicitly or executing correctly. Implement a real TaskBuilder for catalogue calls, or reject call: catalog with a clear unsupported error until it is implemented.
pkg/zigflow/tasks/task_builder_call_catalog.go, pkg/zigflow/tasks/task_builder.go, pkg/zigflow/tasks/task_builder_do.go

Medium: The checked-in JSON schema for CallCatalog is inconsistent with the Go schema and YAML schema. It requires name, but does not declare name as an evaluated property, and it does not require call. With unevaluatedProperties: false, schema consumers can reject valid catalogue tasks or accept invalid shapes. Regenerate docs/static/schema.json from the same source as schema.yaml and add a test that the published schemas validate the new example.
docs/static/schema.json

I could not run the full test suite in this sandbox because the configured Go 1.26.3 toolchain needed a network download, and network access is restricted.