test(cc-config): cover the env runClaudePlugin actually spawns with

`autoInstallSkillPlugins` takes `run` as a seam, so every existing test
stops short of the real `spawnSync` — the line that decides whether
CLAUDE_CONFIG_DIR is pinned was untested. Restoring the unconditional
pin here (the bug that sent `.claude.json` to `~/.claude/.claude.json`)
kept the whole suite green.

Export `runClaudePlugin` and drive it against a fake `claude` on PATH
that prints the variable back: one test for CLAUDE_HOME set, one for
unset. Mutation-checked — the unconditional pin fails the second.

server/lib/skill-plugins.js

@@ -220,4 +220,8 @@ module.exports = {
   requiredSkillPlugins,
   missingSkillPlugins,
   autoInstallSkillPlugins,
+  // Exported only so a test can pin the env it spawns with. Every other test
+  // injects `run` and so never reaches the real spawn — which is how an
+  // unconditional CLAUDE_CONFIG_DIR pin could be restored here unnoticed.
+  runClaudePlugin,
 };