- Replace per-extension closures with shared ExtensionRuntime
- Split context actions: ExtensionContextActions (required) + ExtensionCommandContextActions (optional)
- Rename LoadedExtension to Extension, remove setter methods
- Change runner.initialize() from options object to positional params
- Derive hasUI from uiContext presence (no separate param)
- Add warning when extensions override built-in tools
- RPC and print modes now provide full command context actions
BREAKING CHANGE: Extension system types and initialization API changed.
See CHANGELOG.md for migration details.
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