fix(plugins): replace dependency marker files with a real satisfaction check

The .dependencies_installed hash-marker system only tracked "was this exact
requirements.txt hashed before" — not whether the packages it names are
actually present. That made it fragile (a wiped venv, a manually removed
package, or a lost/corrupted marker forces a needless full pip reinstall or,
worse, a false skip) and produced dead weight for the ~10 plugins whose
requirements.txt is comment-only (they still paid a pip subprocess on first
boot before a marker existed).

Replace it with requirements_are_satisfied() in plugin_loader.py, which
checks each real requirement line against importlib.metadata directly, so
install_dependencies() only shells out to pip when something is actually
missing or version-mismatched. Drops the marker file entirely: removed all
marker read/write sites in plugin_loader.py and store_manager.py, the
now-pointless marker-cleanup step in the git-update path, the unused legacy
marker implementation in plugin_manager.py, and the already-stale
clear_dependency_markers.sh script.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

test/test_plugin_system.py

@@ -84,8 +84,9 @@ class TestPluginLoader:
     
     def test_dependency_check(self):
         """Test dependency checking logic."""
-        # This would test _check_dependencies_installed and _install_plugin_dependencies
-        # which requires mocking subprocess calls and file operations
+        # Covered by test_plugin_loader.py's install_dependencies tests,
+        # which exercise requirements_has_real_deps/requirements_are_satisfied
+        # and the pip subprocess fallback.
 
 
 class TestPluginExecutor: