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/agent/index.ts

@@ -112,4 +112,9 @@ export const agentMode: Mode = {
     // Minimal fallback - repository is a string in PreparedContext
     return `Repository: ${context.repository}`;
   },
+
+  getSystemPrompt() {
+    // Agent mode doesn't need additional system prompts
+    return undefined;
+  },
 };