Add strict mode for plugin discovery to surface load failures
Problem
`PluginRegistry.discover()` logs warnings when a plugin fails to load but doesn't prevent app startup. This is correct for production resilience, but during development it silently hides import errors, missing dependencies, and broken plugins.
Solution
Add a `--strict` flag (CLI) / `PYRITE_STRICT_PLUGINS=true` (env) that causes plugin discovery to raise on any load failure. Enable by default in `pyrite ci` and test runs.