Fix create_or_join to check for existing session before auto-suffixing (#11)

The auto-suffix feature caused create_or_join to create a new suffixed
session instead of joining the existing one. Check if the session
already exists upfront and return early if so.

Author: justinmoon

src/command_surface.rs

@@ -977,11 +977,15 @@ fn create_or_join(
     create_args: &crate::cli::CreateArgs,
     registry: &WorkflowRegistry,
 ) -> Result<String> {
+    if crate::state::session_dir(&create_args.name)
+        .join("state.json")
+        .exists()
+    {
+        return Ok(create_args.name.clone());
+    }
     match commands::create_with_registry(create_args, registry) {
         Ok(name) => Ok(name),
         Err(err) => {
-            // NOTE: matches the bail! message in state.rs create_session_state()
-            // ("session '{}' already exists"). If that message changes, update here.
             if err.to_string().contains("already exists") {
                 Ok(create_args.name.clone())
             } else {