feat: extend CreateAgentSessionOptions with new properties

- Added systemPrompt for overriding the default system prompt.
- Introduced skills for pre-loaded skills management.
- Added contextFiles for handling pre-loaded context files with path and content attributes.

src/types/pi-coding-agent.d.ts

@@ -4,5 +4,11 @@ declare module "@mariozechner/pi-coding-agent" {
   interface CreateAgentSessionOptions {
     /** Extra extension paths merged with settings-based discovery. */
     additionalExtensionPaths?: string[];
+    /** Override the default system prompt. */
+    systemPrompt?: (defaultPrompt?: string) => string;
+    /** Pre-loaded skills. */
+    skills?: Skill[];
+    /** Pre-loaded context files. */
+    contextFiles?: Array<{ path: string; content: string }>;
   }
 }