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/index.ts

@@ -45,20 +45,24 @@ export type {
 	ContextEvent,
 	ExecOptions,
 	ExecResult,
+	Extension,
+	ExtensionActions,
 	ExtensionAPI,
 	ExtensionCommandContext,
+	ExtensionCommandContextActions,
 	ExtensionContext,
+	ExtensionContextActions,
 	ExtensionError,
 	ExtensionEvent,
 	ExtensionFactory,
 	ExtensionFlag,
 	ExtensionHandler,
+	ExtensionRuntime,
 	ExtensionShortcut,
 	ExtensionUIContext,
 	ExtensionUIDialogOptions,
 	KeybindingsManager,
 	LoadExtensionsResult,
-	LoadedExtension,
 	MessageRenderer,
 	MessageRenderOptions,
 	RegisteredCommand,
@@ -81,6 +85,7 @@ export type {
 	TurnStartEvent,
 } from "./core/extensions/index.js";
 export {
+	createExtensionRuntime,
 	discoverAndLoadExtensions,
 	ExtensionRunner,
 	isBashToolResult,