feat(skills): hifi prototype for dt-coach

Track the implementation of the high-fidelity prototype skill for DT Coach.

## Summary

Introduces a new experimental skill providing a structured framework for building and evaluating high-fidelity, local-only prototypes as measurable experiments. Standardizes prototype creation with experiment-driven development, clear success criteria, and repeatable workflows.

## Key Deliverables

- `SKILL.md` — core skill definition (philosophy, constraints, workflow, validation checklist, troubleshooting)
- `references/stack-reference.md` — setup commands per stack (HTML/JS, Python, Node.js, .NET), storage options, simulation strategies, telemetry guidance
- `references/templates.md` — experiment cards, session reports, experiment summaries
- `references/workflow.md` — 6-step checkpoint-driven workflow (define → setup → implement → instrument → execute → report)

## Related

Closes with #1340