fix: include Odoo core addons path - #20

Summary

• Include both source-checkout addon roots in generated addons_path: ${OE_HOME_EXT}/odoo/addons and ${OE_HOME_EXT}/addons.
• Apply the same corrected path order to Enterprise installs and the later UPGRADE_TO_ENTERPRISE mode.
• Document why both paths are kept and add regression coverage.

Dependency

• Stacked on fix: bound apt lock waits #19 / fix/apt-lock-timeout so this remains a small reviewable installer-hardening slice.

Test Plan

• python -m unittest tests.test_installer_features.InstallerFeatureTests.test_addons_path_includes_odoo_core_and_standard_addons tests.test_installer_features.InstallerFeatureTests.test_enterprise_addons_path_keeps_custom_addons_available tests.test_installer_features.InstallerFeatureTests.test_installer_supports_idempotent_enterprise_upgrade_mode -> passed
• python -m unittest discover -s tests -> passed
• scripts/validate.sh -> passed
• git diff --check -> passed