fix(cli): improve error message when adk web is run in wrong directory

Enhances the AgentLoader error message to help users who run 'adk web'
from incorrect directories. This issue affected multiple teams during
internal workshops when getting started with ADK.

Changes:
- Adds visual directory structure diagram to error message
- Includes explicit 'adk web <agents_dir>' usage example
- Detects when user is inside an agent directory and provides
  targeted hint to run 'adk web .' from parent directory

Fixes #3195

---

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

Co-Authored-By: Claude <[email protected]>

Author: pantsjj

src/google/adk/cli/utils/agent_loader.py

@@ -224,13 +224,32 @@ def _perform_load(self, agent_name: str) -> Union[BaseAgent, App]:
       return root_agent
 
     # If no root_agent was found by any pattern
+    # Check if user might be in the wrong directory
+    hint = ""
+    if os.path.isfile(os.path.join(agents_dir, "agent.py")) or os.path.isfile(
+        os.path.join(agents_dir, "root_agent.yaml")
+    ):
+      hint = (
+          "\n\nHINT: It looks like you might be running 'adk web' from inside an"
+          " agent directory. Try running 'adk web .' from the parent directory"
+          " that contains your agent folder, not from within the agent folder"
+          " itself."
+      )
+
     raise ValueError(
         f"No root_agent found for '{agent_name}'. Searched in"
         f" '{actual_agent_name}.agent.root_agent',"
         f" '{actual_agent_name}.root_agent' and"
-        f" '{actual_agent_name}/root_agent.yaml'. Ensure"
-        f" '{agents_dir}/{actual_agent_name}' is structured correctly, an .env"
-        " file can be loaded if present, and a root_agent is exposed."
+        f" '{actual_agent_name}/root_agent.yaml'."
+        f"\n\nExpected directory structure:"
+        f"\n  <agents_dir>/"
+        f"\n    {actual_agent_name}/"
+        f"\n      agent.py (with root_agent) OR"
+        f"\n      root_agent.yaml"
+        f"\n\nThen run: adk web <agents_dir>"
+        f"\n\nEnsure '{agents_dir}/{actual_agent_name}' is structured correctly,"
+        " an .env file can be loaded if present, and a root_agent is exposed."
+        f"{hint}"
     )
 
   def _ensure_app_name_matches(