Guard MergeNB against merge.tool=nbdime and provide terminal-ready remediation

[Copilot]

MergeNB currently cannot safely operate in the same merge flow when Git is configured with merge.tool=nbdime. This PR adds an explicit compatibility check and gives users copy/paste terminal guidance to disable that configuration before resolving notebook conflicts.

• Activation-time compatibility guard

  • Added ensureSupportedMergeTool(...) in gitIntegration.ts.
  • It reads git config --get merge.tool for the active repo and detects nbdime.
  • On detection, it throws a dedicated UnsupportedMergeToolError.

• User-facing remediation path

  • Added a one-time VS Code error prompt for the repo root when nbdime is detected.
  • Prompt action opens a terminal prefilled with pasteable commands to unset merge.tool, remove mergetool.nbdime config, and optionally uninstall nbdime.

• Early wiring in extension lifecycle

  • Hooked the guard into activate(...) in extension.ts so users are notified immediately, before entering MergeNB conflict resolution flow.

• Implementation details

  • Uses repo-scoped deduping (Set) to avoid repeated warnings in the same workspace.
  • Non-fatal handling for unset merge.tool (expected git config --get exit behavior).

const { stdout } = await execAsync('git config --get merge.tool', { cwd: gitRoot });
const mergeTool = stdout.trim().toLowerCase();

if (mergeTool === 'nbdime') {
  const error = new UnsupportedMergeToolError(gitRoot, mergeTool);
  await showUnsupportedMergeToolGuidance(error);
  throw error;
}

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Original prompt

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This section details on the original issue you should resolve

<issue_title>On use, check if mergetool == nbdime and throw error in gitIntegration.ts.</issue_title>
<issue_description>Give directions to unset it/uninstall as a pastable popup in terminal</issue_description>

<agent_instructions>Follow the guidelines, and incorporate something similar as a check on activating nbdime.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes On use, check if mergetool == nbdime and throw error in gitIntegration.ts. #48

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[Avni2000]

Some intricacies the AI's not aware of -

1. git config --list is global. git config --local --list is local
2. We can unset with regex from "jupyter/ipynb/nbdiff/nbdime" etc. with git config --global (or --local) -- unset {command}
3. We may not even have to do all this stuff about asking to have people paste it (though it's a good source of friction, kinda like sudo) if we just check the git API in vscode or custom make one. (this is complicated)

[Avni2000]

We should also make a proper test for this, since I know for a fact it'll be the source of a lot of complaints