Add seed-existing-repo.sh to retrofit CMD into an existing repository

[quackerjeff]

Summary

Add a companion bootstrap script for applying CMD scaffolding to an already-created repository.

Problem

scripts/seed-repo.sh is optimized for creating a new repository from scratch. Many real teams will already have service repositories and need a safe way to add the CMD workflow to those repos without recreating them.

Proposed enhancement

Add scripts/seed-existing-repo.sh that:

• targets an existing repository directory
• validates that the target is already a git repository or offers a clear mode for handling a non-git directory
• applies a selected repo profile such as ui, api, worker, data, fullstack, or platform
• copies the shared CMD assets listed in the manifests
• creates local bootstrap files when missing
• avoids overwriting existing files without explicit confirmation or flags
• reports which files were created, skipped, or would conflict

Expected benefit

• easier adoption in brownfield repos
• safer rollout of CMD across multi-repo systems
• less manual copying and lower risk of inconsistent repo setup
• better path for gradually standardizing existing service repos

Acceptance criteria

• seed-existing-repo.sh supports the same repo types as seed-repo.sh
• the script detects and reports file conflicts clearly
• the script does not overwrite existing files by default
• bootstrap docs explain when to use seed-repo.sh vs seed-existing-repo.sh
• the resulting repo includes the same local bootstrap structure expected by CMD