**Replace main.py with module-based entry point and unify CLI execution**

* Remove legacy *main.py* and introduce *pkgmgr* module entry via *python -m pkgmgr*
* Add ***main**.py* as the canonical entry point delegating to the CLI
* Export *PYTHONPATH=src* in Makefile to ensure reliable imports in dev and CI
* Update setup scripts (venv & nix) to use module execution
* Refactor all E2E tests to execute the real module entry instead of file paths

This aligns pkgmgr with standard Python packaging practices and simplifies testing, setup, and execution across environments.

https://chatgpt.com/share/693c9056-716c-800f-b583-fc9245eab2b4

tests/e2e/test_tools_help.py

@@ -18,14 +18,6 @@ import sys
 import unittest
 from typing import List
 
-
-# Resolve project root (the repo where main.py lives, e.g. /src)
-PROJECT_ROOT = os.path.abspath(
-    os.path.join(os.path.dirname(__file__), "..", "..")
-)
-MAIN_PATH = os.path.join(PROJECT_ROOT, "main.py")
-
-
 def _run_main(argv: List[str]) -> None:
     """
     Helper to run main.py with the given argv.
@@ -40,7 +32,7 @@ def _run_main(argv: List[str]) -> None:
     try:
         sys.argv = ["pkgmgr"] + argv
         try:
-            runpy.run_path(MAIN_PATH, run_name="__main__")
+            runpy.run_module("pkgmgr", run_name="__main__")
         except SystemExit as exc:  # argparse uses this for --help
             # SystemExit.code can be int, str or None; for our purposes:
             code = exc.code