chore: migrate self-distribution links from GitHub to self-hosted Gitea

GitHub accounts suspended; all first-party distribution now served from
git.881218.xyz/rogue/NexusGate (Gitea):
- raw:  /rogue/NexusGate/raw/branch/<branch>/<path>
- release assets and source archives under the same repo path
- scripts gain a GIT_BASE variable built from NG_REPO (default
  rogue/NexusGate); NG_REPO/NG_BRANCH/--repo/--branch semantics unchanged
- panel enrollment/uninstall commands in public/app.js point at Gitea raw
- install.sh/nexusgate.sh/agent-singbox.sh stats source previously
  defaulted to the upstream repo (a2899882); defaults now this repo
Upstream dependency references (sagernet/sing-box Go module) untouched.

agent-go/INSTALL-GO.md

@@ -7,8 +7,8 @@ Agent 已从 Node.js（`agent/agent.js`）切换为 Go 静态二进制
 
 ## 新装（scripts/agent-install.sh）
 
-- 默认从 GitHub Release 下载二进制：
-  `https://github.com/aaaamoooo/NexusGate/releases/download/agent-v0.6.8/nexusgate-agent-linux-<arch>`
+- 默认从 Gitea Release 下载二进制：
+  `https://git.881218.xyz/rogue/NexusGate/releases/download/agent-v0.6.8/nexusgate-agent-linux-<arch>`
   （`<arch>` 由 `uname -m` 决定：x86_64→amd64，aarch64→arm64）
 - 安装到 `/opt/nexusgate-agent/nexusgate-agent`（0755）。
 - 服务名仍为 `nexusgate-agent`：