**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

Makefile

@@ -30,6 +30,7 @@ export BASE_IMAGE_CENTOS
 # PYthon Unittest Pattern
 TEST_PATTERN	:= test_*.py
 export TEST_PATTERN
+export PYTHONPATH := src
 
 # ------------------------------------------------------------
 # System install
@@ -45,7 +46,7 @@ install:
 # Default: keep current auto-detection behavior
 setup: setup-nix setup-venv
 
-# Explicit: developer setup (Python venv + shell RC + main.py install)
+# Explicit: developer setup (Python venv + shell RC + install)
 setup-venv: setup-nix
 	@bash scripts/setup/venv.sh