chore: structural refactor and dead code cleanup

[Huerte]

• Bug fix
• New feature
• Documentation
• Refactor / internal

Overview

Split the git_operations.py module into three focused modules (git_core, git_branch, git_remote), relocated misplaced CLI handlers into the commands/ directory, standardized naming across all files and entry points, and removed dead code.

Changes

• git_operations.py: split into git_core.py (commit, init, push), git_branch.py (branch queries), git_remote.py (remote sync, rebase handling).
• utils/config.py: removed config_operation. Moved to commands/config.py.
• main.py: removed user_management and display_current_user. Moved to commands/user.py.
• stash_operation.py -> stash.py, platform_utils.py -> platform.py, setup.py -> bootstrap.py.
• All command entry points renamed to *_operation pattern: state_operation, undo_operation, user_operation.
• validate_repo_url moved from commands/link.py to utils/validators.py.
• Deleted dead code: get_status_content(), create_main_branch(), unused json import in jump.py, 11 installer-only functions in platform.py.
• CONTRIBUTING.md: update the project structure.
• Test files split and updated to match new module layout.

How to Test

1. pip install -e ".[dev]"
2. pytest
3. Expected: 199 passed, 0 failed.

Checklist

• I tested my changes locally and they work
• I updated CHANGELOG.md under the [Unreleased] section
• I updated README.md (if I added or changed a command)
• I added or updated tests for my change (if applicable)
• My change does not break any existing commands