Add GitHub release installation for plugins

[Copilot]

Plugins can now be installed directly from GitHub releases. Users input a repository URL, select a release, choose an asset file, and install.

Backend Changes

New endpoints:

• POST /api/v1/plugins/github/releases - Parse repo URL, fetch releases via GitHub API
• POST /api/v1/plugins/github/release-assets - Fetch assets for selected release
• POST /api/v1/plugins/install/github - Install from asset URL with metadata tracking

All requests use configured proxy. Release metadata (owner, repo, tag, URL) stored in install_info.

Frontend Changes

Multi-step dialog flow:

1. Repository URL input with validation (github.com/owner/repo regex)
2. Release selection (displays tag, name, date, pre-release badge)
3. Asset selection (displays filename, size with auto-formatting)
4. Confirmation with full install details

UI integration:

• "GitHub安装" option in install dropdown
• Back navigation between steps
• Existing "From GitHub" badge displays for installed plugins with repo link
• i18n support: zh-Hans, en-US, zh-Hant, ja-JP

SDK Dependency

Requires adding GITHUB = 'github' to PluginInstallSource enum in langbot-plugin-sdk. Installation handler must download from install_info['asset_url'] and store GitHub metadata.

# langbot_plugin/runtime/plugin/mgr.py
class PluginInstallSource(Enum):
    LOCAL = 'local'
    MARKETPLACE = 'marketplace'
    GITHUB = 'github'  # Required addition

Limitations

• Public repositories only (no authentication)
• GitHub API rate limits apply (60 req/hour unauthenticated)
• Draft releases filtered out

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• registry.npmmirror.com
  • Triggering command: npm install (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

目前插件安装只支持 本地上传 和 插件市场 安装，有时候有些插件作者会仅发布到github，我们需要支持从github安装；

安装步骤如下：

1. 用户在插件管理页面选择 “GitHub安装”
2. 弹出对话框要求输入仓库地址
3. 后端根据仓库地址获取仓库releases中的各个release，用户选择release，在同一对话框中展示该release中的各个文件，选中并确认后可以安装

注意：需要添加一种新的安装标记：来自GitHub
注意：你可能需要同时修改 langbot-app/langbot-plugin-sdk 仓库的代码

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[codecov]

Codecov Report

❌ Patch coverage is 7.46269% with 62 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/api/http/controller/groups/plugins.py	3.92%	49 Missing ⚠️
pkg/plugin/connector.py	18.75%	13 Missing ⚠️

📢 Thoughts on this report? Let us know!