feat: add plugin loader infrastructure

Implement plugin loading and validation:
- loader.py: discover, import, validate plugins
- validate PLUGIN_MANIFEST, MODEL_REQUIREMENTS, create_engine
- validate api_version compatibility (major must match)
- validate capabilities (reject unknown)
- diagnostic messages for all error cases
- no partial registration after error

Test plugins:
- fake_plugin: minimal valid plugin for testing
- missing_manifest: no PLUGIN_MANIFEST
- invalid_api_version: major version mismatch
- invalid_capabilities: unknown capabilities
- missing_create_engine: no create_engine function
- import_error: raises ImportError during import
- missing_model_requirements: no MODEL_REQUIREMENTS

39 new tests covering all loader functionality.

tests/plugins/missing_manifest/__init__.py

@@ -0,0 +1,10 @@
+"""Invalid plugin: missing PLUGIN_MANIFEST."""
+
+from __future__ import annotations
+
+# This plugin intentionally does NOT export PLUGIN_MANIFEST
+MODEL_REQUIREMENTS: list = []
+
+
+def create_engine(context, model_path, config):
+    raise NotImplementedError("This plugin is invalid")