feat: list agents on health endpoint

[mattjohnsonpint]

Lists the active agents on the /health endpoint, formatted for readability.

Example output:

{
  "status": "ok",
  "environment": "dev",
  "version": "v0.18.0-alpha.3-5-g9566a5cf",
  "agents": [
    {"id": "d0ro44tj5qn5rbq37ong", "name": "Counter", "status": "running"},
    {"id": "d0ro44tj5qn5rbq37opg", "name": "Counter", "status": "running"},
    {"id": "d0ro44lj5qn5rbq37olg", "name": "Counter", "status": "running"}
  ]
}

[linear]

HYP-3423 Report agents on health endpoint

[Copilot]

Pull Request Overview

This PR introduces a new feature to list agents on the /health endpoint by including an "agents" array in the JSON response, as well as refactoring agent-related types to be exported with appropriate JSON field annotations.

• Added JSON formatting for the /health endpoint response to include a list of active agents
• Refactored internal agent types to exported AgentInfo and AgentStatus
• Updated the CHANGELOG with the new feature

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
runtime/httpserver/health.go	Updates the /health endpoint to output formatted JSON including an "agents" array
runtime/actors/agents.go	Updates agent types to be exported and adds ListAgents helper function
CHANGELOG.md	Documents the addition of listing agents on the /health endpoint

[trunk-io]

Running Code Quality on PRs by uploading data to Trunk will soon be removed. You can still run checks on your PRs using trunk-action - see the migration guide for more information.