Built-in agents (Sisyphus, Hephaestus, Atlas) now include explicit identity section in their system prompts, matching custom agent behavior. 12 agent identity tests pass, 266 total pass, tsc clean. Closes #2964
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent) Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>