feat: add review mode for PR code reviews

- Add 'review' as a new execution mode in action.yml
- Use default GitHub Action token (ACTIONS_TOKEN) for review mode
- Create review mode implementation with GitHub MCP tools included by default
- Move review-specific prompt to review mode's generatePrompt method
- Add comprehensive review workflow instructions for inline comments
- Fix type safety with proper mode validation
- Keep agent mode's simple inline prompt handling

src/modes/registry.ts

@@ -13,11 +13,12 @@
 import type { Mode, ModeName } from "./types";
 import { tagMode } from "./tag";
 import { agentMode } from "./agent";
+import { reviewMode } from "./review";
 import type { GitHubContext } from "../github/context";
 import { isAutomationContext } from "../github/context";
 
 export const DEFAULT_MODE = "tag" as const;
-export const VALID_MODES = ["tag", "agent"] as const;
+export const VALID_MODES = ["tag", "agent", "review"] as const;
 
 /**
  * All available modes.
@@ -26,6 +27,7 @@ export const VALID_MODES = ["tag", "agent"] as const;
 const modes = {
   tag: tagMode,
   agent: agentMode,
+  review: reviewMode,
 } as const satisfies Record<ModeName, Mode>;
 
 /**