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/pull-request-target.test.ts

@@ -1,38 +1,10 @@
 #!/usr/bin/env bun
 
 import { describe, test, expect } from "bun:test";
-import {
-  getEventTypeAndContext,
-  generatePrompt,
-  generateDefaultPrompt,
-} from "../src/create-prompt";
+import { getEventTypeAndContext, generatePrompt } from "../src/create-prompt";
 import type { PreparedContext } from "../src/create-prompt";
-import type { Mode } from "../src/modes/types";
 
 describe("pull_request_target event support", () => {
-  // Mock tag mode for testing
-  const mockTagMode: Mode = {
-    name: "tag",
-    description: "Tag mode",
-    shouldTrigger: () => true,
-    prepareContext: (context) => ({ mode: "tag", githubContext: context }),
-    getAllowedTools: () => [],
-    getDisallowedTools: () => [],
-    shouldCreateTrackingComment: () => true,
-    generatePrompt: (context, githubData, useCommitSigning) =>
-      generateDefaultPrompt(context, githubData, useCommitSigning),
-    prepare: async () => ({
-      commentId: 123,
-      branchInfo: {
-        baseBranch: "main",
-        currentBranch: "main",
-        claudeBranch: undefined,
-      },
-      mcpConfig: "{}",
-      claudeArgs: "",
-    }),
-  };
-
   const mockGitHubData = {
     contextData: {
       title: "External PR via pull_request_target",
@@ -126,12 +98,7 @@ describe("pull_request_target event support", () => {
         },
       };
 
-      const prompt = generatePrompt(
-        envVars,
-        mockGitHubData,
-        false,
-        mockTagMode,
-      );
+      const prompt = generatePrompt(envVars, mockGitHubData, false, "tag");
 
       // Should contain pull request event type and metadata
       expect(prompt).toContain("<event_type>PULL_REQUEST</event_type>");
@@ -165,12 +132,7 @@ describe("pull_request_target event support", () => {
         },
       };
 
-      const prompt = generatePrompt(
-        envVars,
-        mockGitHubData,
-        false,
-        mockTagMode,
-      );
+      const prompt = generatePrompt(envVars, mockGitHubData, false, "tag");
 
       // Should include git commands for non-commit-signing mode
       expect(prompt).toContain("git push");
@@ -196,7 +158,7 @@ describe("pull_request_target event support", () => {
         },
       };
 
-      const prompt = generatePrompt(envVars, mockGitHubData, true, mockTagMode);
+      const prompt = generatePrompt(envVars, mockGitHubData, true, "tag");
 
       // Should include commit signing tools
       expect(prompt).toContain("mcp__github_file_ops__commit_files");
@@ -246,13 +208,13 @@ describe("pull_request_target event support", () => {
         pullRequestContext,
         mockGitHubData,
         false,
-        mockTagMode,
+        "tag",
       );
       const pullRequestTargetPrompt = generatePrompt(
         pullRequestTargetContext,
         mockGitHubData,
         false,
-        mockTagMode,
+        "tag",
       );
 
       // Both should have the same event type and structure
@@ -293,37 +255,7 @@ describe("pull_request_target event support", () => {
         },
       };
 
-      // Use agent mode which passes through the prompt as-is
-      const mockAgentMode: Mode = {
-        name: "agent",
-        description: "Agent mode",
-        shouldTrigger: () => true,
-        prepareContext: (context) => ({
-          mode: "agent",
-          githubContext: context,
-        }),
-        getAllowedTools: () => [],
-        getDisallowedTools: () => [],
-        shouldCreateTrackingComment: () => true,
-        generatePrompt: (context) => context.prompt || "default prompt",
-        prepare: async () => ({
-          commentId: 123,
-          branchInfo: {
-            baseBranch: "main",
-            currentBranch: "main",
-            claudeBranch: undefined,
-          },
-          mcpConfig: "{}",
-          claudeArgs: "",
-        }),
-      };
-
-      const prompt = generatePrompt(
-        envVars,
-        mockGitHubData,
-        false,
-        mockAgentMode,
-      );
+      const prompt = generatePrompt(envVars, mockGitHubData, false, "agent");
 
       expect(prompt).toBe(
         "Review this pull_request_target PR for security issues",
@@ -343,12 +275,7 @@ describe("pull_request_target event support", () => {
         },
       };
 
-      const prompt = generatePrompt(
-        envVars,
-        mockGitHubData,
-        false,
-        mockTagMode,
-      );
+      const prompt = generatePrompt(envVars, mockGitHubData, false, "tag");
 
       // Should generate default prompt structure
       expect(prompt).toContain("<event_type>PULL_REQUEST</event_type>");
@@ -418,7 +345,7 @@ describe("pull_request_target event support", () => {
 
       // Should not throw when generating prompt
       expect(() => {
-        generatePrompt(minimalContext, mockGitHubData, false, mockTagMode);
+        generatePrompt(minimalContext, mockGitHubData, false, "tag");
       }).not.toThrow();
     });
 
@@ -476,13 +403,13 @@ describe("pull_request_target event support", () => {
         internalPR,
         mockGitHubData,
         false,
-        mockTagMode,
+        "tag",
       );
       const externalPrompt = generatePrompt(
         externalPR,
         mockGitHubData,
         false,
-        mockTagMode,
+        "tag",
       );
 
       // Should have same tool access patterns