fix: remove references to deleted install-host-agent.sh script

The unified agent system replaced install-host-agent.sh with install.sh.
This commit updates all references:
- Dockerfile: removed COPY for deleted script
- router.go: serve install.sh at /install-host-agent.sh endpoint (backwards compatible)
- build-release.sh: removed copy of deleted script
- validate-release.sh: removed validation of deleted script
- install.sh: updated script list for bare-metal installs

install.sh

@@ -2737,12 +2737,12 @@ deploy_agent_scripts() {
     local scripts=(
         "install-docker-agent.sh"
         "install-container-agent.sh"
-        "install-host-agent.sh"
         "install-host-agent.ps1"
         "uninstall-host-agent.sh"
         "uninstall-host-agent.ps1"
         "install-sensor-proxy.sh"
         "install-docker.sh"
+        "install.sh"
     )
 
     local deployed=0