chore: point CODEOWNERS at @letta-ai/mods-owners

[just-cameron]

Summary

• Collapse the duplicated Charles+Caren CODEOWNERS rules into a single * @letta-ai/mods-owners line so required review is OR, not two individual requests.
• Team members: @cpacker, @carenthomas, @just-cameron. Any one of them satisfies code-owner review.

GitHub still auto-requests every user listed on a CODEOWNERS line, which is why Charles stayed requested after Caren approved #80. A team is one review request; any member counts.

Needs admin: grant @letta-ai/mods-owners write on this repo. GitHub ignores a team in CODEOWNERS until the team itself has write, even if every member already has it personally. I do not have admin on mods, so I could not do that from here.

Test plan

• After merge, open a PR that touches any file and confirm the requested reviewer is the mods-owners team, not both Charles and Caren as individuals
• Confirm one approval from Charles, Caren, or Cameron satisfies the code-owner check