chore: categorize empty repo error [CM-1103]

[mbani01]

This pull request adds support for handling empty repositories in the git integration service. The main changes introduce a new error type for empty repositories, update the error code enumeration, and ensure that ambiguous HEAD errors are mapped to this new error. The most important changes are:

Error handling improvements:

• Added a new error code EMPTY_REPO to the ErrorCode enum in enums.py, representing the case when a repository is empty (no branches or commits).
• Introduced a new EmptyRepoError dataclass in errors.py to encapsulate the error message and code for empty repositories.
• Updated the error mapping in utils.py so that ambiguous HEAD errors (which indicate an empty repository) now raise EmptyRepoError.
• Added EmptyRepoError to the list of imported errors in utils.py to enable its use in error handling logic.

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Note

Low Risk
Low risk: adds a new error code/exception and maps a specific git stderr pattern to it, only affecting how empty-repo failures are classified and reported.

Overview
Adds first-class classification for empty repositories in the git integration service.

Introduces ErrorCode.EMPTY_REPO and a corresponding EmptyRepoError, and updates shell error classification so git failures containing ambiguous argument 'HEAD' are surfaced as EmptyRepoError instead of a generic command failure.

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[Copilot]

Pull request overview

Adds a dedicated error classification for empty Git repositories to the git integration service so “ambiguous HEAD” failures can be surfaced with a specific error code.

Changes:

• Added EMPTY_REPO to ErrorCode.
• Introduced EmptyRepoError as a new CrowdGitError type.
• Classified ambiguous HEAD stderr output to raise EmptyRepoError in shell error handling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
services/apps/git_integration/src/crowdgit/services/utils.py	Maps ambiguous HEAD stderr to the new EmptyRepoError.
services/apps/git_integration/src/crowdgit/errors.py	Adds EmptyRepoError dataclass with standardized message/code.
services/apps/git_integration/src/crowdgit/enums.py	Extends ErrorCode with EMPTY_REPO.

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