feat: accept Debian release numbers

[slegarraga]

Closes #29

Summary

Adds Debian release-number support alongside codenames for fetch-debian and list-debian.

For example, --release 12 now resolves to bookworm exactly as --release bookworm does. Unknown numbers fail with the same KconfigSymbolNotFoundError shape used for Ubuntu release resolution.

Tests

Added tests/test_debian_release.py covering:

• fetch-debian resolves release number 12 to bookworm pockets;
• list-debian resolves release number 12 to the same pockets.

Verification

uv run ruff check kconfig/cli/kernel.py tests/test_debian_release.py
uv run mypy kconfig/cli/kernel.py
uv run pytest tests/test_debian_release.py -q --no-cov

All pass.

Signed off with DCO.