Add workspace file changes field definition and resolver to Workspace GraphQL type
MR: Pending <!-- NOTE: For context on MR heading, see: https://handbook.gitlab.com/handbook/engineering/devops/dev/create/remote-development/index.html#relationship-of-issues-to-mrs --> ## Description As part of our [new designs for Workspaces](https://gitlab.com/groups/gitlab-org/-/epics/18674), we want the the frontend to display uncommitted file changes in expanded workspace cards to help users identify workspaces with unsaved work. To achieve this, we need to add a `changedFiles` field to the existing Workspace GraphQL type and implement its resolver. ## Acceptance criteria - [ ] Add `fileChanges` field definition to existing Workspace GraphQL type - [ ] Implement resolver method that returns information about locally modified files compared to the active branch - [ ] Include change count and file paths for uncommitted changes - [ ] Add GraphQL field documentation with examples ## TODO: Implementation plan ## Technical Notes **Current Limitation:** As [noted by](https://gitlab.com/gitlab-org/gitlab/-/issues/563085#note_2702314222) @cwoolley-gitlab, we currently don't have the architecture to support this sort of visibility into the current state of the repo in a running workspace. We will need a way to sync workspace git status with Rails monolith. <!-- NOTE: Feel free to expand with more sections and headers as needed --> <!-- DO NOT TOUCH BELOW -->
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