feat(docs): simplify custom GitHub App creation with manifest support (#620)

* feat(docs): simplify custom GitHub App creation with manifest support

- Add github-app-manifest.json with pre-configured permissions
- Create interactive HTML tool for one-click app creation
- Update setup.md documentation with manifest-based instructions
- Maintain existing manual setup as alternative option

This significantly improves the developer experience by eliminating
manual permission configuration and reducing setup time from multiple
steps to a single click.

Fixes #619

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Kris Coleman <kriscodeman@gmail.com>

* feat: create-app ux improvements

Signed-off-by: Kris Coleman <kriscodeman@gmail.com>

---------

Signed-off-by: Kris Coleman <kriscodeman@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

github-app-manifest.json

@@ -0,0 +1,27 @@
+{
+  "name": "Claude Code Custom App",
+  "description": "Custom GitHub App for Claude Code Action - AI-powered coding assistant for GitHub workflows",
+  "url": "https://github.com/anthropics/claude-code-action",
+  "hook_attributes": {
+    "url": "https://example.com/github/webhook",
+    "active": false
+  },
+  "redirect_url": "https://github.com/settings/apps/new",
+  "callback_urls": [],
+  "setup_url": "https://github.com/anthropics/claude-code-action/blob/main/docs/setup.md",
+  "public": false,
+  "default_permissions": {
+    "contents": "write",
+    "issues": "write",
+    "pull_requests": "write",
+    "actions": "read",
+    "metadata": "read"
+  },
+  "default_events": [
+    "issue_comment",
+    "issues",
+    "pull_request",
+    "pull_request_review",
+    "pull_request_review_comment"
+  ]
+}