feat: support GITMOB_CONFIG_FILE for storing git-mob settings (non-breaking)

[rkotze]

Adds support for using an environment variable (GITMOB_CONFIG_FILE) to store git-mob settings in a custom git config file instead of the global config. When GITMOB_CONFIG_FILE is set to a path, git-mob will use git config --file … for its settings; otherwise it continues to use --global. This is non-breaking and intended for per-machine/dotfiles workflows (see issue #282).

Changes:

• New helper: packages/git-mob-core/src/git-mob-api/config-scope.ts
• Use helper in packages/git-mob-core/src/git-mob-api/git-mob-config.ts
• Tests: packages/git-mob-core/test/git-mob-config.spec.ts

Notes:

• GITMOB_CONFIG_FILE should be an absolute/expanded path (no automatic ~ expansion).
• local repo settings still use --local as before.
• Optionally we can add README docs on the new env var.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Updated the CHANGELOG.md to capture my changes
• Build (npm run build) was successfully run locally
• All tests and linting (npm run checks) has passed locally
• I kept my pull requests small so it can be reviewed easier

Pull request type

Please check the type of change your PR introduces:

• Feature

Does this introduce a breaking change?

• No

Other information