fix: suppress stderr during parent package loading

Fixes "No module named 'agents'" error shown during startup when using
fully qualified agent paths that include intermediate package directories.

Issue: When loading agents from paths like /path/agents/my_agent.py,
the loader tries to import parent packages. If those __init__.py files
have import errors, Python prints error messages to stderr even though
the errors are caught and handled.

Solution:
- Added contextlib.redirect_stderr() to suppress error output during
  parent package initialization
- Errors are still caught and logged at DEBUG level for troubleshooting
- Agent loading still works correctly (errors are non-critical)

Impact:
- Cleaner startup experience with no confusing error messages
- Agent functionality unchanged
- All 161 tests passing

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

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

Author: jwesleye

src/basic_agent_chat_loop/components/agent_loader.py

@@ -4,7 +4,9 @@
 Handles dynamic loading of agent modules and extraction of agent metadata.
 """
 
+import contextlib
 import importlib.util
+import io
 import logging
 import os
 import sys
@@ -115,12 +117,17 @@ def _ensure_package_loaded(package_root: Path, package_name: str) -> None:
         # Register in sys.modules BEFORE executing
         sys.modules[pkg] = pkg_module
 
+        # Suppress stderr during package init - parent packages in agent paths
+        # often have import errors that are non-critical (agent still works)
         try:
-            spec.loader.exec_module(pkg_module)
+            with contextlib.redirect_stderr(io.StringIO()):
+                spec.loader.exec_module(pkg_module)
         except Exception as e:
-            # If package init fails, still keep it registered
-            # (might just have imports that will work later)
-            logger.debug(f"Package {pkg} __init__.py had errors: {e}")
+            # If package init fails, still keep it registered (might just have
+            # imports that will work later). This commonly happens with
+            # intermediate directories in agent paths (e.g., /path/agents/my_agent.py)
+            if logger.isEnabledFor(logging.DEBUG):
+                logger.debug(f"Package {pkg} __init__.py initialization failed: {type(e).__name__}")
             pass