fix: validate tool download destination before network

[lukisch]

Summary

download_tool() contacted GitHub for release metadata before checking whether the local destination was usable. A regular file or a destination whose metadata check raises OSError could therefore produce a network error instead of the local destination error.

This change checks the destination and existing parents before any network request, keeps the existing Download error: cannot prepare destination: prefix, and leaves existing destination contents untouched. The later mkdir remains as the race-time guard.

Validation

• Destination preflight tests: 2 passed.
• tests/test_tool_manager.py: 26 passed.
• Full suite: 104 passed, 6 subtests passed.
• Ruff, Python AST parse, and git diff --check: passed.