fix: auto-assign after merge, persistent server logs, remove duplicate pnpm install

- Call auto_assign_available_work at end of merge_agent_work so the next
  story gets picked up without waiting for the PTY exit handler
- Add persistent file logging to .story_kit/logs/server.log so server
  logs survive restarts
- Remove duplicate pnpm install block in run_squash_merge

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/main.rs

@@ -100,6 +100,13 @@ async fn main() -> Result<(), std::io::Error> {
         }
     }
 
+    // Enable persistent server log file now that the project root is known.
+    if let Some(ref root) = *app_state.project_root.lock().unwrap() {
+        let log_dir = root.join(".story_kit").join("logs");
+        let _ = std::fs::create_dir_all(&log_dir);
+        log_buffer::global().set_log_file(log_dir.join("server.log"));
+    }
+
     let workflow = Arc::new(std::sync::Mutex::new(WorkflowState::default()));
 
     // Filesystem watcher: broadcast channel for work/ pipeline changes.