fix(agent): improve backward compat for PBS-only hosts. Related to #925

The legacy state file could represent either PVE or PBS registration,
depending on what was installed at the time. Now we check what's
currently installed to determine the correct behavior:
- If PVE is installed: legacy file means PVE was registered
- If PBS-only (no PVE): legacy file means PBS was registered

internal/hostagent/proxmox_setup.go

@@ -510,20 +510,50 @@ func (p *ProxmoxSetup) isAlreadyRegistered() bool {
 
 // isTypeRegistered checks if a specific Proxmox type has been registered.
 func (p *ProxmoxSetup) isTypeRegistered(ptype string) bool {
-	// Check per-type state file
+	// Check per-type state file first (new behavior)
 	stateFile := p.stateFileForType(ptype)
 	if _, err := os.Stat(stateFile); err == nil {
 		return true
 	}
 
-	// Also check legacy state file for backward compat
-	// If the legacy file exists and matches this type, consider it registered
+	// Check legacy state file for backward compat
 	if _, err := os.Stat(stateFilePath); err == nil {
-		// Legacy file exists - only the first detected type was registered
-		// For backward compat, treat PVE as registered if legacy file exists
-		// (since PVE was always detected first in the old logic)
+		// Legacy file exists. The old detection logic was:
+		// 1. If pvesh exists → registered "pve"
+		// 2. Else if proxmox-backup-manager exists → registered "pbs"
+		//
+		// So we need to figure out what was likely registered.
+		// If PVE is currently installed, treat PVE as registered.
+		// If only PBS is installed (no PVE), treat PBS as registered.
+		types := p.detectProxmoxTypes()
+		if len(types) == 0 {
+			return false
+		}
+
+		// The old code registered the "first" type it found
+		// If PVE is installed, it was always detected first
 		if ptype == "pve" {
-			return true
+			for _, t := range types {
+				if t == "pve" {
+					return true
+				}
+			}
+		}
+		// If PBS-only host (no PVE), PBS was what was registered
+		if ptype == "pbs" {
+			hasPVE := false
+			hasPBS := false
+			for _, t := range types {
+				if t == "pve" {
+					hasPVE = true
+				}
+				if t == "pbs" {
+					hasPBS = true
+				}
+			}
+			if hasPBS && !hasPVE {
+				return true
+			}
 		}
 	}