fix: preserve workspace gitignore entries

[fzlzjerry]

Summary

• preserve existing workspace .gitignore content during force-init and update
• append only missing Pythonlings-managed patterns in their declared order
• keep repeated init and update operations idempotent

Closes #43

Tests

• python -m pytest -q (Python 3.11: 150 passed)
• python -m pytest tests/unit/test_curriculum.py tests/integration/test_cli_workspace.py -q (Python 3.13: 13 passed)

Screenshots

Not applicable; this changes filesystem behavior without changing CLI output.

Checklist

• Documentation is unchanged because the commands and managed patterns are unchanged
• Added or updated tests
• Verified python -m pytest -q

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