Move linter and diff utils to openhands-aci

[ryanhoangt]

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• [ ] Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

This PR is to:

• [x] Port the linter and its relevant utils (e.g. diff, shell command execution) into openhands-aci
• [x] ~~Enable linter for the editor and run eval~~ --> the result is slightly worse than the baseline. More details in discussion here: https://openhands-ai.slack.com/archives/C080M7BBSSG/p1731546156509479 --> We'll disable the linter by default.

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Link of any specific issues this addresses