* fix(skills): create a missing extensions_config.json in the project root
When no extensions config resolved, the skill toggle created one at
Path.cwd().parent while the MCP writers used project_root(). Unless the
Gateway runs from backend/ with the project root at its parent, the
toggle's file landed outside the resolver's search path: the toggle was
lost on reload and the next MCP edit created a second file.
Add ExtensionsConfig.default_config_path() (the project root, the first
location resolve_config_path searches) and use it from the skill toggle
and both MCP create paths, so a created file is always the one the next
resolution reads back.
* docs(changelog): reference #7048 for the extensions config create path
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
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