Fix viewer-only installs registering as agents

Previously, any installation with the protocol handler registered
would default to running as an agent. This caused admin/technician
machines (viewer-only) to appear in the sessions list.

Changes:
- Add Config::has_agent_config() to check for explicit agent config
- Only run as agent when: explicit 'agent' command, support code
  provided, OR agent config file exists
- Viewer-only installs (protocol handler but no config) now exit
  silently when launched without arguments

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/src/config.rs

@@ -105,6 +105,31 @@ impl Default for EncodingConfig {
 }
 
 impl Config {
+    /// Check if an explicit agent configuration file exists
+    /// This returns true only if there's a real config file, not generated defaults
+    pub fn has_agent_config() -> bool {
+        // Check for config in current directory
+        let local_config = PathBuf::from("guruconnect.toml");
+        if local_config.exists() {
+            return true;
+        }
+
+        // Check in program data directory (Windows)
+        #[cfg(windows)]
+        {
+            if let Ok(program_data) = std::env::var("ProgramData") {
+                let path = PathBuf::from(program_data)
+                    .join("GuruConnect")
+                    .join("agent.toml");
+                if path.exists() {
+                    return true;
+                }
+            }
+        }
+
+        false
+    }
+
     /// Load configuration from file or environment
     pub fn load() -> Result<Self> {
         // Try loading from config file