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/docs/sdk.md

@@ -784,15 +784,18 @@ interface CreateAgentSessionResult {
   // The session
   session: AgentSession;
   
-  // Custom tools (for UI setup)
-  customToolsResult: {
-    tools: LoadedCustomTool[];
-    setUIContext: (ctx, hasUI) => void;
-  };
+  // Extensions result (for runner setup)
+  extensionsResult: LoadExtensionsResult;
   
   // Warning if session model couldn't be restored
   modelFallbackMessage?: string;
 }
+
+interface LoadExtensionsResult {
+  extensions: Extension[];
+  errors: Array<{ path: string; error: string }>;
+  runtime: ExtensionRuntime;
+}
 ```
 
 ## Complete Example