Add seamless migration path from legacy agents to unified agent:
- Add AgentType field to report payloads (unified vs legacy detection)
- Update server to detect legacy agents by type instead of version
- Add UI banner showing upgrade command when legacy agents are detected
- Add deprecation notice to install-host-agent.ps1
- Create install-docker-agent.sh stub that redirects to unified installer
Legacy agents (pulse-host-agent, pulse-docker-agent) now show a "Legacy"
badge in the UI with a one-click copy command to upgrade to the unified
agent.