feat(Makefile): add install target with existing-version guard

[Josh-Jing]

What this does | 这个 PR 做了什么

English:
This PR adds a proper make install target that automates from-source installation:

1. Builds all binaries from source (reasonix + reasonix-plugin-example)
2. Installs them to $(PREFIX)/bin (defaults to ~/.local/bin)
3. Adds an existing-version guard: if an installation already exists in the target path, it errors out and asks the user to run make install FORCE=1 to overwrite.

Why this helps:

• Provides a one-step installation path for users who want to build from source. Previously users had to manually cp binaries to PATH, which could be error-prone and left users wondering if all components were installed correctly.
• The guard prevents accidental overwrites of an existing working installation (e.g. the npm-installed binary)
• Motivation: I personally ran into this issue today: after following the Quick Start and running npm i -g reasonix, I got a 0.x (legacy TypeScript) version instead of 1.x (Go). The configuration didn't work, and I had to manually copy the built binaries to PATH - which made me wonder if I had all required components installed properly. This PR solves that experience gap.
• Related: there is already PR fix(npm): auto-publish stable 1.x releases under latest tag #4352 addressing the npm dist-tag issue. This PR complements that by making it easier for users who want to install from source directly.

Updated both README.md and README.zh-CN.md to document the new target.

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中文说明:
本 PR 添加了完整的 make install 目标，自动化从源码安装流程：

1. 从源码构建所有二进制文件（reasonix + reasonix-plugin-example）
2. 安装到 $(PREFIX)/bin（默认 ~/.local/bin）
3. 增加已有版本防护：如果目标路径已经存在 reasonix 安装，会报错提示需要使用 make install FORCE=1 才允许覆盖。

解决的问题：

• 为从源码安装的用户提供一步到位的安装命令，之前用户需要手动 cp 二进制到 PATH，容易出错，而且会担心是不是漏装了组件。现在一条命令搞定，安装路径清晰易管理。
• 防护机制防止在用户没有明确要求的情况下，意外覆盖已有的安装（比如通过 npm 安装的版本）。
• 动机：我自己今天就踩坑了：按照 README 的 Quick Start 执行 npm i -g reasonix，结果装完是 0.x TypeScript 旧版本，按照文档配置后完全不生效。排查之后不得不从源码构建再手动复制到 PATH，但过程中一直怀疑是不是漏装了某个组件。这个 PR 填补了这个体验缺口。
• 相关：已有 PR fix(npm): auto-publish stable 1.x releases under latest tag #4352 解决 npm 标签问题，本 PR 是补充，让想直接从源码安装的用户更省心。

同时更新了英文和简体中文 README 文档。

Testing | 测试

• ✅ make install 全新安装正常工作
• ✅ make install 在已有安装时报错提示
• ✅ make install FORCE=1 成功覆盖安装
• ✅ 同时安装 reasonix 和 reasonix-plugin-example 两个二进制
• ✅ 安装完成后提示检查 PATH 环境变量