Add prime-agent as a selectable/default-able agent harness option

[mbravorus]

prime-agent is an RLM-based self-improving agent harness based on pi, see https://github.com/PrimeIntellect-ai/prime-agent

I would like to suggest adding it as an option to the agent-picker. The nitpick is that it needs a non-mise install path (requires pulling a private tarball bundle from R2 plus curl bootstrap, which in turn necessitates handling the update path), but after that it launches flag-free just like pi.

I feel it is a worthy and powerful harness option, and volunteer to submit a well-tested PR if this warrants enough maintainer attention.