Agent removes incompatible packages but doesn't prompt for replacement

### Description

Direct customer feedback:

> When upgrading a .NET 6 application to .NET 10, the agent removed an APM package because it doesn't support .NET 10, but did not ask what to replace it with. The customer uses a different APM provider as their replacement, but the agent had no way to know that.

Expected Behavior

The agent should prompt the user when removing a dependency that has no compatible version, asking whether they want to specify an alternative. Ideally, org-level dependency replacement mappings could be configured (e.g., "if removing PackageA, suggest PackageB").

Additional Context:

Related to #1688 (resolving to specified versions) but distinct — this is specifically about prompting for alternatives when a package is being removed entirely due to incompatibility.