Refactor templates engine out of the core CLI

The templating engine currently lives inside the core CLI binary, which
forces every template change to ship with a CLI release and bloats the
thin CLI. Move the engine into its own package (or per-workload
component, per the templating ADR) so templates can iterate
independently. Unblocks Go / Durable / Kubernetes workloads.

## Definition of Done

- [ ] Templating engine lives outside the core CLI (location per the
templating ADR).
- [ ] Existing template-driven commands (`func new`, `func init`)
continue to work end to end.
- [ ] Existing template tests pass against the extracted engine.
- [ ] Core CLI no longer references the engine implementation directly,
only its public contract.
- [ ] Docs updated (`building-a-workload.md` or successor) to describe
the new engine location.