docs: add starter template boilerplate for new 12-factor agents

[Bortlesboat]

Summary

• add new content/starter-template.md with project layout, v0 checklist, and framework-agnostic starter loop
• add a prominent README tip linking to the starter template

Why

Issue/discussion traffic asks for a practical template/boilerplate path. This provides a concise entry point without changing core factor content.

Validation

• markdown-only change; links and paths verified locally

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