refactor: centralize editor plugin lifecycle ownership

CONTEXT.md

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+# vamp
+
+A modal editor whose features are independent of the frontend used to present them.
+
+## Language
+
+**Editor session**:
+An active editing environment containing a buffer, its modal state, and the plugins available to that environment.
+_Avoid_: plugin manager (when referring to the whole editing environment)
+
+**Feature**:
+A named set of editor commands with declared frontend compatibility requirements.
+
+**Plugin**:
+An extension that contributes a feature to an editor session. A failed plugin can become unavailable without ending the session.
+
+**Core feature**:
+A protected feature that provides essential editing behavior and cannot be replaced or disabled by an extension.
+
+**Frontend**:
+The presentation and input experience through which a user interacts with an editor session.
+
+**Capability**:
+A named frontend ability required by a feature for compatibility, not a permission granted to that feature.