test(import[prune]) Add edge-case tests for prune scoping

[tony]

Summary

• Add 3 new prune edge-case tests covering previously untested scenarios
• Test --prune without import_source does nothing (Finding 13)
• Test --sync --prune together is idempotent with --sync alone (Finding 14)
• Test same-name same-source repos across workspaces prune correctly (Finding 1)

Test plan

• uv run py.test -vvv — all tests pass
• uv run mypy — clean
• uv run ruff check . — clean

[tony]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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