feat: add Git repository check and automatic .gitignore append safety

[sandman-sh]

Feature Request

When setting up TestSprite in a project, there are two common onboarding safety needs:

1. Git Tracking: Agent skills require Git tracking to safely manage code modifications. We should warn the user if they are not in a Git repository.
2. Gitignore Safety: The .testsprite/ folder stores local run artifacts and test execution logs. We should automatically ignore it in .gitignore (during testsprite setup) or check and warn if it is not ignored (during testsprite doctor) to avoid committing these files to version control.

[sandman-sh]

/assign

[testsprite-hob]

Assigned to @sandman-sh. Thanks for taking this on. (2 more slots available.)

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