fix: clarify custom build hook path errors

[oiahoon]

Summary

• clarify that missing relative custom build-hook paths are resolved from the project root
• preserve the existing diagnostic for absolute paths
• cover both path forms with focused regression tests

Closes #1226

Verification

• pytest tests/backend/builders/hooks/test_custom.py (7 passed)
• pytest tests/backend (942 passed, 49 skipped)
• ruff check and ruff format --check on both changed files
• isolated Hatchling sdist and wheel build
• git diff --check