docs: refresh template README

[DeyangChan]

Summary

• Reframe the template README around a clearer product promise and faster start path.
• Explain how base contracts, one stack pack, and chosen add-ons compose a project.
• State explicitly that the template reflects Cavalry Collective's design and software engineering philosophy, not a universal standard.
• Preserve the Day-1 setup, workflow, support, security, and licensing guidance.

Spec: N/A — documentation-only refresh with no product behaviour change.

Test plan

• Repository document and contract checks report 0 problem(s).
• git diff --check passes.
• Every relative README link resolves through the repository integrity check.
• The changed README was reviewed end to end against the live Visual Stack README.

Notes

• New abstraction / dependency / config key: none added.
• Pattern followed: the current Cavalry-Collective/visual-stack README's product-led structure, adapted to this template's purpose.