First-class Prime Agent launch and session restore

[lucasrgt]

Prime Agent 0.7.0 already includes a built-in Herdr reporter. When it runs in a Herdr 0.8.0 pane, herdr agent list correctly shows prime-agent and its lifecycle state without installing the Pi integration.

The remaining gap is that Herdr does not recognize prime-agent as a supported agent kind. In a clean disposable session I observed:

1. launching prime-agent --offline creates a visible idle prime-agent through its reporter;
2. herdr agent explain returns agent_explain_unavailable because there is no detected agent label;
3. herdr agent start --kind does not offer prime-agent;
4. after a clean /quit, the pane can remain labelled prime-agent after the foreground process has returned to the shell;
5. after a Herdr server restart, the pane returns as a shell instead of resuming the reported Prime session.

Prime Agent supports prime-agent --resume <path|id> and its reporter already supplies a session path or ID. Treating Prime Agent as a distinct supported agent would complete launch, identity, cleanup, and cold session restore while retaining the built-in reporter as the lifecycle authority.

Would first-class Prime Agent support fit Herdr's direction? If so, could a maintainer define the preferred contribution path or approve an implementation issue for it?