docs: converge Code Locator research and point to active repository

[BestNathan]

Summary

Converge the Code Locator research line inside Narness without modifying the active code-exploration repository.

• mark the in-repo Code Locator implementation as an archived historical snapshot;
• distill the durable findings from the later System One localization experiments into the Progressive Action Spaces topic;
• separate the completed Code Locator case study from the still-active broader progressive-action-space research;
• point active implementation and ongoing research to BestNathan/system-one-code-explore;
• make the migration visible from the root README and topics index;
• retire the old system-one-code-locator workflow so Narness no longer creates new Code Locator experiment records.

Research boundary

Narness keeps the architectural conclusions and provenance. It no longer mirrors active Code Locator implementation, benchmark branches, workflows, or experiment logs.

The workflow briefly produced one offline evidence record while this migration branch was being prepared; that generated record was removed after the workflow was retired.

Related historical PRs: #16, #17, #21, #22, #23, #24, #25.