feat: add and handle diff_opts.on_unsaved_changes option

[derethil]

Graceful Dirty Buffer Handling for Diff Operations

This PR adds the ability to gracefully handle unsaved changes when creating diffs. This has two behaviors to choose from:

1. error (default): Throws an error when attempting to create a diff with unsaved changes. This is unchanged from the existing behavior.
2. discard: Automatically discards unsaved buffer changes by reloading the buffer from disk before creating the diff.

The implementation includes:

• A new diff_opts.on_unsaved_changes configuration option with validation
• A new discard_buffer_changes() function that safely reloads buffers from disk
• Integration with the existing diff setup workflow
• Comprehensive test coverage for both error and discard behaviors

This makes it easier to work with diffs when files have unsaved changes, allowing you to choose between safety or convenience based on your workflow preferences.

[ThomasK33]

Hey @derethil, thanks for this PR.

It's mostly looking good. The only thing missing is the update to lua/claudecode/types.lua.

The others are minor issues that you can choose to address if you want :)

[derethil]

Changes pushed up in the latest commits - great idea on the discard warning, thanks for that suggestion.