Feature request: parent-directory MCP scoping

Currently, .mcp.json is only loaded from the immediate project root. There's no way to define MCPs that apply to all projects under a parent folder without putting them in user scope (which makes them global to everything).

Use case: A developer has two folder groups — e.g. ~/sites/talabat/* and ~/sites/projects/* — each needing a different set of MCP servers. Claude is always opened from a leaf project directory, never the parent. There's no way to express "all projects under talabat/ share these MCPs" without either duplicating .mcp.json in every project or polluting user scope.

Proposed behaviour: Claude Code walks up the directory tree from the project root, loading any .mcp.json files it finds in ancestor directories, with closer-to-root files taking lower precedence (so the project's own .mcp.json wins on conflicts). This mirrors how tools like ESLint, .gitignore, and EditorConfig handle config inheritance.