feat: add dynamic default branch resolution

[Huerte]

Type

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

Overview

The gitgo link and init commands use your actual default branch. It stops hardcoding "master". It checks your global init.defaultBranch first. Then it checks gitgo.default-branch. If both are missing, it just uses main.

Changes

• config.py: Added get_default_branch() to run git config --get init.defaultBranch.
• git_core.py & link.py: Replaced the hardcoded fallbacks with get_default_branch().
• tests: Added test_get_default_branch_* for the native config, gitgo config, and hard fallback.
• CHANGELOG.md: Added the new feature to the Unreleased section.

How to Test

1. pip install -e ".[dev]"
2. Set a custom git default branch: git config --global init.defaultBranch custom-branch
3. Run gitgo link in a new empty repo.
4. Expected result: GitGo initializes the repo using custom-branch instead of main or master.

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 (if it does, describe the impact in the Overview)