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.

.changeset/two-bats-smoke.md

@@ -0,0 +1,5 @@
+---
+"task-master-ai": patch
+---
+
+Split monolithic task-master-core.js into separate function files within