refactor: simplify mode system by removing Mode interface and registry (#899)

Replace the over-engineered Mode interface/registry/detector pattern with
straightforward inline logic. There are only 2 modes (tag and agent) and
the complexity wasn't justified.

- Delete Mode interface, registry, and prepare pass-through modules
- Export prepareTagMode() and prepareAgentMode() as standalone functions
- Inline trigger checking and mode dispatch in run.ts/prepare.ts
- Change generatePrompt/createPrompt to take modeName string instead of Mode
- Remove dead code (extractGitHubContext, unused detector helpers)
- Update CLAUDE.md to reflect new architecture

test/modes/tag.test.ts

@@ -1,92 +1,8 @@
-import { describe, test, expect, beforeEach } from "bun:test";
-import { tagMode } from "../../src/modes/tag";
-import type { ParsedGitHubContext } from "../../src/github/context";
-import type { IssueCommentEvent } from "@octokit/webhooks-types";
-import { createMockContext } from "../mockContext";
+import { describe, test, expect } from "bun:test";
+import { prepareTagMode } from "../../src/modes/tag";
 
 describe("Tag Mode", () => {
-  let mockContext: ParsedGitHubContext;
-
-  beforeEach(() => {
-    mockContext = createMockContext({
-      eventName: "issue_comment",
-      isPR: false,
-    });
-  });
-
-  test("tag mode has correct properties", () => {
-    expect(tagMode.name).toBe("tag");
-    expect(tagMode.description).toBe(
-      "Traditional implementation mode triggered by @claude mentions",
-    );
-    expect(tagMode.shouldCreateTrackingComment()).toBe(true);
-  });
-
-  test("shouldTrigger delegates to checkContainsTrigger", () => {
-    const contextWithTrigger = createMockContext({
-      eventName: "issue_comment",
-      isPR: false,
-      inputs: {
-        ...createMockContext().inputs,
-        triggerPhrase: "@claude",
-      },
-      payload: {
-        comment: {
-          body: "Hey @claude, can you help?",
-        },
-      } as IssueCommentEvent,
-    });
-
-    expect(tagMode.shouldTrigger(contextWithTrigger)).toBe(true);
-
-    const contextWithoutTrigger = createMockContext({
-      eventName: "issue_comment",
-      isPR: false,
-      inputs: {
-        ...createMockContext().inputs,
-        triggerPhrase: "@claude",
-      },
-      payload: {
-        comment: {
-          body: "This is just a regular comment",
-        },
-      } as IssueCommentEvent,
-    });
-
-    expect(tagMode.shouldTrigger(contextWithoutTrigger)).toBe(false);
-  });
-
-  test("prepareContext includes all required data", () => {
-    const data = {
-      commentId: 123,
-      baseBranch: "main",
-      claudeBranch: "claude/fix-bug",
-    };
-
-    const context = tagMode.prepareContext(mockContext, data);
-
-    expect(context.mode).toBe("tag");
-    expect(context.githubContext).toBe(mockContext);
-    expect(context.commentId).toBe(123);
-    expect(context.baseBranch).toBe("main");
-    expect(context.claudeBranch).toBe("claude/fix-bug");
-  });
-
-  test("prepareContext works without data", () => {
-    const context = tagMode.prepareContext(mockContext);
-
-    expect(context.mode).toBe("tag");
-    expect(context.githubContext).toBe(mockContext);
-    expect(context.commentId).toBeUndefined();
-    expect(context.baseBranch).toBeUndefined();
-    expect(context.claudeBranch).toBeUndefined();
-  });
-
-  test("getAllowedTools returns empty array", () => {
-    expect(tagMode.getAllowedTools()).toEqual([]);
-  });
-
-  test("getDisallowedTools returns empty array", () => {
-    expect(tagMode.getDisallowedTools()).toEqual([]);
+  test("prepareTagMode is exported as a function", () => {
+    expect(typeof prepareTagMode).toBe("function");
   });
 });