feat: add an existing-project adoption prompt and guide

[nothingnesses]

Summary

• Add an adoption prompt to the default pack and its generated reference copy.
• Add a tested guide for existing projects, with human approval before instruction changes or reference replacement.
• Shorten the README and separate detailed references from introductory guidance.
• Add regression tests for adoption commands and preservation of project material.

Runtime behaviour and existing delivery-role contracts remain unchanged. The guide distinguishes the unreleased workflow from the published 0.0.4 crate.

Related to #11.

Validation

• Full host just ci passed.
• All five focused scaffold and adoption tests passed.
• Source installation into a private prefix passed.
• The installed binary passed the displayed adoption commands and a repeat-run rehearsal.
• Independent product review returned PASS with no findings.

The tests check shipped instructions and command behaviour. They do not prove that a harness follows the prompt or correctly interprets project intent.