refactor(mcp): Modularize direct functions in MCP server

Split monolithic task-master-core.js into separate function files within
the mcp-server/src/core/direct-functions/ directory. This change:

- Creates individual files for each direct function implementation
- Moves findTasksJsonPath to a dedicated utils/path-utils.js file
- Converts task-master-core.js to be a simple import/export hub
- Improves maintainability and organization of the codebase
- Reduces potential merge conflicts when multiple developers contribute
- Follows standard module separation patterns

Each function is now in its own self-contained file with clear imports and
focused responsibility, while maintaining the same API endpoints.

.cursor/rules/changeset.mdc

@@ -15,7 +15,7 @@ Changesets is used to manage package versioning and generate accurate `CHANGELOG
     - **Bug Fixes** (Fixes to existing functionality)
     - **Breaking Changes** (Changes that are not backward-compatible)
     - **Performance Improvements** (Enhancements to speed or resource usage)
-    - **Significant Refactoring** (Major code restructuring, even if external behavior is unchanged, as it might affect stability or maintainability)
+    - **Significant Refactoring** (Major code restructuring, even if external behavior is unchanged, as it might affect stability or maintainability) - *Such as reorganizing the MCP server's direct function implementations into separate files*
     - **User-Facing Documentation Updates** (Changes to README, usage guides, public API docs)
     - **Dependency Updates** (Especially if they fix known issues or introduce significant changes)
     - **Build/Tooling Changes** (If they affect how consumers might build or interact with the package)