cli: reject init --force at the curriculum source #71

Motivation

Running forced initialization from a source checkout can crash after Pythonlings resolves the destination to the same directory as its bundled curriculum. A contributor-facing command should reject that target before attempting any copy.

Current evidence

Deep QA on dev at e1d4dae37234b75124bd6a53f06734296ebbbdee reproduced this twice in disposable copies:

PYTHONPATH=<source-copy> python -m pythonlings init --path <source-copy> --force
shutil.SameFileError: <source-copy>/info.toml and <source-copy>/info.toml are the same file

The command exits 1 with a traceback. init_workspace() starts copying before checking whether path equals source_root(). update_workspace() already rejects the equivalent source target.

Exact scope

Reject forced initialization when the resolved destination is the curriculum source, before any file is copied or removed. Return a concise pythonlings: error through the existing CLI error path.

Acceptance checklist

• init --path <curriculum-source> --force exits non-zero without a traceback.
• The source checkout is unchanged after the rejected command.
• The error identifies that the curriculum source cannot be initialized in place.
• Forced initialization of a different existing workspace retains its current behavior.
• First initialization and repeated non-forced initialization retain their current behavior.
• Regression coverage exercises the resolved same-path case.

Explicit exclusions

Do not change update target precedence, overwrite semantics for a normal learner workspace, curriculum copy layout, or automatic workspace discovery.

Likely files

• pythonlings/core/curriculum.py
• pythonlings/cli.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
pythonlings --root tests/fixtures/passing_curriculum verify

Repeat the same-path command against a disposable source copy and confirm its tree hash is unchanged.

Sprint coordination

See Discussion #37 and tracker #52. This issue is intentionally unassigned. A coordination comment is welcome, but it does not reserve the issue. The first pull request that satisfies every acceptance criterion and verification requirement is considered for merge.