research: remove migrated Code Locator payload

[BestNathan]

Summary

Finish the Code Locator migration cleanup inside Narness.

• remove the old Code Locator source implementation;
• remove tests and fixture repository content;
• remove the old design and pilot documents;
• remove all persisted Code Locator run artifacts and traces;
• reduce research/code-locator/ to a small migration README;
• update the root README and Progressive Action Spaces topic so they no longer describe the directory as an archived implementation snapshot.

Result

research/code-locator/ now contains only README.md.

This removes 72 obsolete files / about 41.5 MB from the current tree. Historical provenance remains available through Git history and the closed research PRs. Active implementation and experiment records remain in BestNathan/system-one-code-explore.