feat: add getSystemPrompt method to mode interface (#400)

Allows modes to provide custom system prompts that are appended to Claude's base system prompt. This enables mode-specific instructions without modifying the core action logic.

- Add optional getSystemPrompt method to Mode interface
- Implement method in all existing modes (tag, agent, review)
- Update prepare.ts to call getSystemPrompt and export as env var
- Wire up APPEND_SYSTEM_PROMPT in action.yml to pass to base-action

All modes currently return undefined (no additional prompts), but the infrastructure is now in place for future modes to provide custom instructions.

src/modes/types.ts

@@ -73,6 +73,13 @@ export type Mode = {
    * @returns PrepareResult with commentId, branchInfo, and mcpConfig
    */
   prepare(options: ModeOptions): Promise<ModeResult>;
+
+  /**
+   * Returns an optional system prompt to append to Claude's base system prompt.
+   * This allows modes to add mode-specific instructions.
+   * @returns The system prompt string or undefined if no additional prompt is needed
+   */
+  getSystemPrompt?(context: ModeContext): string | undefined;
 };
 
 // Define types for mode prepare method