fix: narrow lockfile plugin CI selection

[lucarlig]

Summary

• keep Cargo.lock from forcing all plugins when plugin-specific paths identify the changed plugin set
• preserve conservative all-plugin selection for lockfile-only changes
• add regression coverage for Cargo.lock plus one plugin path

Fixes #80

Validation

• python3 -m unittest tests.test_plugin_catalog.PluginCatalogTests.test_ci_selection_treats_cargo_lock_change_as_all_plugins tests.test_plugin_catalog.PluginCatalogTests.test_ci_selection_keeps_cargo_lock_with_plugin_change_plugin_scoped
• python3 tools/plugin_catalog.py validate .
• python3 tools/plugin_catalog.py ci-selection . diff a028daee84c0879bda1a38b59111afecbf044b87 c628d106327bf595c024d7fe515abcab7d7ff903