fix: detect xlings package path and fallback MCPP_HOME to ~/.mcpp (#60)

When mcpp is installed via `xlings install mcpp`, the binary lives at
~/.xlings/data/xpkgs/xim-x-mcpp/<ver>/bin/mcpp. The previous
home_dir() detected bin/ and used the grandparent (the xpkgs package
directory) as MCPP_HOME, creating a nested xlings sandbox that broke
toolchain installation and lost installed toolchains on mcpp upgrade.

Now home_dir() checks for the data/xpkgs ancestor pattern and falls
back to ~/.mcpp/ (same behavior as CI with explicit MCPP_HOME).

Also extract default_mcpp_home() with unified Windows (%USERPROFILE%)
and POSIX ($HOME) fallback logic.

Author: Sunrisepeak

.agents/docs/fix-xlings-package-home-detection.md

@@ -0,0 +1,24 @@
+# Fix: xlings 包内 mcpp 的 MCPP_HOME 检测
+
+## 问题
+
+`xlings install mcpp` 安装的 mcpp 二进制位于 `~/.xlings/data/xpkgs/xim-x-mcpp/0.0.20/bin/mcpp`。
+`home_dir()` 检测到 `bin/` 父目录后，将祖父目录作为 MCPP_HOME，导致嵌套沙箱：
+
+```
+MCPP_HOME = ~/.xlings/data/xpkgs/xim-x-mcpp/0.0.20/
+  → registry/ = ~/.xlings/data/xpkgs/xim-x-mcpp/0.0.20/registry/
+    → XLINGS_HOME = 同上
+      → 工具链安装到 registry/data/xpkgs/xim-x-llvm/20.1.7/  ← 嵌套失败
+```
+
+## 修复
+
+在 `home_dir()` 的自包含模式检测中，增加 xlings 包路径排除：如果祖先目录中存在
+`data/xpkgs` 模式，说明在 xlings 包内，fallback 到 `~/.mcpp/`。
+
+同时提取 `default_mcpp_home()` 统一三平台的 HOME fallback（Windows 用 %USERPROFILE%）。
+
+## 改动
+
+仅 `src/config.cppm` 的 `home_dir()` 函数，约 20 行。

src/config.cppm

@@ -168,31 +168,47 @@ void print_status(std::string_view verb, std::string_view msg) {
     }
 }
 
+std::filesystem::path default_mcpp_home() {
+    // Windows: %USERPROFILE%\.mcpp   POSIX: $HOME/.mcpp
+    if constexpr (mcpp::platform::is_windows) {
+        if (auto* e = std::getenv("USERPROFILE"); e && *e)
+            return std::filesystem::path(e) / ".mcpp";
+    }
+    if (auto* e = std::getenv("HOME"); e && *e)
+        return std::filesystem::path(e) / ".mcpp";
+    return std::filesystem::current_path() / ".mcpp";
+}
+
 std::filesystem::path home_dir() {
+    // 1. Explicit $MCPP_HOME takes priority (CI, advanced users).
     if (auto* e = std::getenv("MCPP_HOME"); e && *e)
         return std::filesystem::path(e);
 
-    std::error_code ec;
     auto exe = mcpp::platform::fs::self_exe_path();
     if (exe.has_parent_path() && exe.parent_path().filename() == "bin") {
-        // Dev builds emit binaries at target/<triple>/<fp>/bin/<exe>,
-        // matching the bin/ shape. Any ancestor literally named
-        // "target" disqualifies self-contained mode and falls through
-        // to $HOME/.mcpp — so first-run on a dev binary doesn't drop
-        // a half-populated sandbox into target/.
-        bool isDevPath = false;
-        for (auto p = exe.parent_path();
-             !p.empty() && p != p.parent_path();
+        auto candidate = exe.parent_path().parent_path();
+
+        // Disqualify self-contained mode for two cases:
+        //   a) Dev builds: .../target/<triple>/<fp>/bin/<exe>
+        //   b) xlings packages: .../data/xpkgs/xim-x-mcpp/<ver>/bin/mcpp
+        //      Creating a nested xlings sandbox inside the xpkgs directory
+        //      breaks toolchain installation (nested XLINGS_HOME) and loses
+        //      installed toolchains when the mcpp package version is upgraded.
+        bool disqualified = false;
+        for (auto p = candidate;
+             p.has_parent_path() && p != p.root_path();
              p = p.parent_path()) {
-            if (p.filename() == "target") { isDevPath = true; break; }
+            if (p.filename() == "target") { disqualified = true; break; }
+            if (p.filename() == "xpkgs") {
+                auto parent = p.parent_path().filename().string();
+                if (parent == "data") { disqualified = true; break; }
+            }
         }
-        if (!isDevPath)
-            return exe.parent_path().parent_path();
+        if (!disqualified)
+            return candidate;
     }
 
-    if (auto* e = std::getenv("HOME"); e && *e)
-        return std::filesystem::path(e) / ".mcpp";
-    return std::filesystem::current_path() / ".mcpp";
+    return default_mcpp_home();
 }
 
 std::expected<std::string, std::string> run_capture(const std::string& cmd) {