fix: prevent panic when tracing subscriber already initialized

Use try_init() instead of init() for tracing subscriber setup to
gracefully handle cases where a global subscriber is already set.

This fixes a panic in the scout agent subprocess when spawned by the
research tool, where a dependency may have already initialized tracing.

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

crates/g3-cli/src/utils.rs

@@ -169,8 +169,8 @@ pub fn initialize_logging(verbose: bool) {
             .add_directive("llama=off".parse().unwrap())
     };
 
-    tracing_subscriber::registry()
+    let _ = tracing_subscriber::registry()
         .with(tracing_subscriber::fmt::layer())
         .with(filter)
-        .init();
+        .try_init();
 }