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;
}

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

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