fix: correct type annotations for Plugin and event_handler

[wu-vincent]

Summary

• Fix Plugin class variable types — were typed as bare None, causing mypy [assignment] errors when subclasses assign strings (e.g. prefix = "MyPlugin")
• Fix event_handler decorator return type — stub returned None, causing mypy [func-returns-value] and [untyped-decorator] errors

These are the two issues that force plugins like endweave to use # type: ignore comments.

Before

class MyPlugin(Plugin):
    prefix = "Test"       # type: ignore[assignment]
    api_version = "0.11"  # type: ignore[assignment]

    @event_handler(priority=EventPriority.LOWEST)  # type: ignore[func-returns-value,untyped-decorator]
    def on_event(self, event: SomeEvent) -> None: ...

After

class MyPlugin(Plugin):
    prefix = "Test"
    api_version = "0.11"

    @event_handler(priority=EventPriority.LOWEST)
    def on_event(self, event: SomeEvent) -> None: ...

Test plan

• mypy passes on a plugin file using prefix = "...", api_version = "...", @event_handler, and @event_handler(priority=...)
• Existing tests unaffected

🤖 Generated with Claude Code