fix: preserve existing workspace .gitignore entries

[abhiksark]

Motivation

Workspace initialization currently rewrites .gitignore with only Pythonlings-managed patterns. That silently discards learner or team ignore rules, including during forced initialization and update.

Current evidence

• pythonlings/core/curriculum.py::_write_workspace_gitignore() writes a replacement file from GITIGNORE_LINES.
• init_workspace() and update_workspace() both call that helper.

Exact scope

Change managed ignore handling to append only missing Pythonlings patterns. Preserve existing user lines exactly: no deletion, duplication, or reordering during init, force-init, or update.

Acceptance checklist

• An existing .gitignore retains every original line in its original order.
• Missing managed patterns are appended once in the declared managed-pattern order.
• Repeated init/update operations are idempotent and do not duplicate managed patterns.
• Empty or absent .gitignore gets the managed patterns.
• Normal init, force-init, and update paths are all covered by tests.

Explicit exclusions

Do not normalize, sort, deduplicate, or otherwise rewrite user-authored ignore rules; do not change the managed patterns themselves.

Likely files

• pythonlings/core/curriculum.py
• tests/unit/test_curriculum.py
• tests/integration/test_cli_workspace.py

Verification

python -m pytest tests/unit/test_curriculum.py tests/integration/test_cli_workspace.py -q
python -m pytest -q

Manually inspect before/after .gitignore fixtures containing comments, blank lines, and an existing managed pattern.

Sprint coordination

See the sprint Discussion. This issue is unassigned and is not a reservation.

[fzlzjerry]

I'd like to take this. I'll update workspace .gitignore handling to preserve existing lines, append only missing managed patterns, and add coverage for normal init, force-init, update, and repeated operations.