Restructure Flutter plugin codebase into flutter/ directory for monorepo

[helin24]

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[gemini-code-assist]

Code Review

This pull request restructures the repository into a monorepo by moving the Flutter plugin codebase, documentation, and agent skills into a new flutter/ subdirectory, updating configuration files like .gitignore and .github/dependabot.yml accordingly. The review feedback suggests correcting the root README.md to accurately describe the repository contents (as the Dart plugin is not present), updating an outdated branch reference in the CODE_OF_CONDUCT.md link, and removing redundant ignore patterns from the .gitignore file.

[gemini-code-assist]

[CONCERN] The README states that this repository is a monorepo containing both the Flutter and Dart plugins. However, there is currently no Dart plugin codebase in this repository (only the flutter/ directory is present). If the Dart plugin is not part of this repository, we should update the description to avoid confusion.

Suggested change

	# Flutter and Dart IntelliJ Plugins
	[](https://plugins.jetbrains.com/plugin/9212-flutter)
	[](https://github.com/flutter/flutter-intellij/actions?query=branch%3Amain+workflow%3Apresubmit)
	This repository is a monorepo containing the IntelliJ IDEA and Android Studio plugins for Flutter and Dart.
	# Flutter IntelliJ Plugin
	This repository is a monorepo containing the IntelliJ IDEA and Android Studio plugins for Flutter.

[gemini-code-assist]

[CONCERN] The link to Flutter's code of conduct uses the outdated master branch. Since the flutter/flutter repository has migrated to main as its default branch, we should update this link to use main to ensure long-term reliability.

Suggested change

	When contributing code to the `flutter-intellij` project, please observe
	Flutter's [code of conduct](https://github.com/flutter/flutter/blob/master/CODE_OF_CONDUCT.md).
	When contributing code to the `flutter-intellij` project, please observe
	Flutter's [code of conduct](https://github.com/flutter/flutter/blob/main/CODE_OF_CONDUCT.md).

[gemini-code-assist]

[CONCERN] The entry flutter/tool/plugin/.idea is redundant because the global pattern **/.idea (defined at line 9) already ignores all .idea directories at any depth. Additionally, **/.kotlin/ at line 48 already covers .kotlin/ at line 47. We should remove these redundant entries to keep the .gitignore file clean and maintainable.

**/.kotlin/