Extract Linux/Debian setup helper

[codeforester]

Summary

• extract the Ubuntu/Debian setup/check surface from setup_common.sh into setup_linux_debian.sh
• preserve existing setup_* function names, platform dispatch, output text, dry-run behavior, and --yes consent behavior
• refresh docs/setup-common-ownership.md so the current ownership map reflects the first extracted platform helper
• extend docs/tests so sourced setup helper anchors and the Linux/Debian source guard stay covered

Decision

The Linux/Debian surface is now cohesive enough to justify a helper boundary:

• apt package prerequisites and dry-run commands
• GitHub CLI official Debian/Ubuntu apt repository setup
• Linux Python finder and runtime tool probes
• Linux check-result collector
• Linux install body

The helper is named setup_linux_debian.sh so the OS/platform scope is explicit.

Fixes #1564.
Refs #1570.

Validation

• /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest tests/test_setup_common_ownership_docs.py tests/test_contract_hardening.py -q
• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/setup-common.bats
• bash -n cli/bash/commands/basectl/subcommands/setup_common.sh cli/bash/commands/basectl/subcommands/setup_linux_debian.sh
• shellcheck -S error cli/bash/commands/basectl/subcommands/setup_common.sh cli/bash/commands/basectl/subcommands/setup_linux_debian.sh
• /Users/rameshhp/.base.d/base/.venv/bin/python -m pylint --rcfile=.pylintrc tests/test_setup_common_ownership_docs.py
• git diff --check
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1570-linux-helper bin/basectl check --ci