Add AGPLv3 LICENSE and initial README for ModAPI

[Milvona]

Motivation

• Add an explicit open-source license to the repository to clarify terms of use and distribution for the project.
• Provide an initial README.md describing the ModAPI project, its features and build instructions to help developers get started quickly.

Description

• Add a LICENSE file containing the full text of the GNU Affero General Public License v3.0 to license the project under AGPL-3.0-or-later.
• Add README.md with an overview of ModAPI, supported features, environment requirements, project structure, and build instructions including xmake f -m release and xmake.
• Document output artifacts and where build products are copied (bin/), and point to the LICENSE from the README.

Testing

• No automated tests were run for this change since it only adds licensing and documentation files.

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Codex Task

[chatgpt-codex-connector]

💡 Codex Review

ModAPI/README.md

Line 9 in 2b47644

- 自定义游戏规则、世界生成特征、战利品表、状态效果和实体接口

Remove claims for missing registry APIs

This feature list advertises custom loot-table, status-effect, and entity APIs as available, but I checked include/ and src/ with rg and the matching files are only empty/stub headers: include/modapi/effect/CustomEffectRegistry.h is empty, while the block/entity/loot-table registry headers only include stub base headers and have no declarations or implementations. Users following the new README will look for APIs that are not present in this release, so these items should be marked as planned or removed until implemented.

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