fix: avoid local task module collisions

[D3f0]

Summary

• load each local_tasks.py under a path-derived private module name
• preserve independent local task collections when multiple directories are loaded
• add regression coverage for distinct local task modules

Verification

• uv run pytest tests/test_collection.py tests/test_program_with_collection.py -q
• uv run ruff check src/invoke_toolkit/collections.py tests/test_collection.py
• uv run pytest -q (670 passed, 1 existing failure in tests/extensions/test_package_template_entrypoint.py caused by Rich line wrapping)
• commit hooks passed except pre-existing pylint warning W0237 in ToolkitCollection.configure

Fixes #122