refactor(coding-agent): simplify extension runtime architecture

- 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.

packages/coding-agent/src/core/index.ts

@@ -26,6 +26,7 @@ export {
 	discoverAndLoadExtensions,
 	type ExecOptions,
 	type ExecResult,
+	type Extension,
 	type ExtensionAPI,
 	type ExtensionCommandContext,
 	type ExtensionContext,
@@ -38,7 +39,6 @@ export {
 	type ExtensionShortcut,
 	type ExtensionUIContext,
 	type LoadExtensionsResult,
-	type LoadedExtension,
 	type MessageRenderer,
 	type RegisteredCommand,
 	type SessionBeforeBranchEvent,