fix: prefer main local task namespace

[D3f0]

Summary

• warn and skip local_tasks.py when the primary collection already owns the local task namespace
• preserve the primary task or alias so task discovery completes
• add regression coverage for the alias collision

Closes #132

Verification

• uv run pytest — 726 passed, 18 skipped
• uv run ruff check src/invoke_toolkit/collections.py tests/test_collection.py
• uv run ty check tests/test_collection.py
• reproduced intk -l in datascape: warning emitted and deploy-local (local) remains available