refactor: replace find-up dependency with internal implementation

[MarshallOfSound]

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• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

This PR removes the external find-up dependency and replaces it with an internal implementation in core-utils. The new findUp function provides the same core functionality—walking up the directory tree to find files—while being simpler and more tailored to the project's needs.

Changes:

• Added src/find-up.ts with an async findUp function that searches for files by walking up the directory tree from a given starting directory
• Added comprehensive test coverage in spec/find-up.spec.ts covering:
  • Finding files in the starting directory
  • Finding files in ancestor directories
  • Returning the closest match when multiple ancestors contain a candidate
  • Respecting the order of candidate names within a single directory
  • Ignoring directories that share a candidate name
  • Returning undefined when no candidate exists
• Updated src/package-manager.ts and src/electron-version.ts to use the new internal implementation
• Updated test mocks in spec/package-manager.spec.ts to mock the internal function
• Removed find-up from package.json dependencies

The new implementation is simpler than the external dependency, removes an external dependency, and provides the exact functionality needed by the codebase.

https://preview.claude.ai/code/session_01VxunirAtie59oe2RTm1DGr