Add Ubuntu prerequisite diagnostics and fix hints

[codeforester]

Summary

• Add Ubuntu/Debian prerequisite diagnostics for Bash, Python venv support, Git, GitHub CLI, BATS, ShellCheck, jq, and Go.
• Keep the checks read-only with command -v, Bash version inspection, and python3 -m venv --help.
• Emit actionable Ubuntu/Debian fix hints without invoking Homebrew or Xcode paths.
• Assign stable BASE-D008 through BASE-D015 finding IDs and document them.

Validation

• bats cli/bash/commands/basectl/tests/check.bats
• bats cli/bash/commands/basectl/tests/doctor.bats
• PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/pytest tests/test_doctor_findings_docs.py cli/python/base_setup/tests/test_diagnostics_payloads.py cli/python/base_setup/tests/test_diagnostics.py
• shellcheck --severity=error cli/bash/commands/basectl/subcommands/setup_common.sh cli/bash/commands/basectl/tests/check.bats cli/bash/commands/basectl/tests/doctor.bats cli/bash/commands/basectl/tests/setup_helpers.bash
• git diff --check
• env -u BASE_HOME BASE_CACHE_DIR=/private/tmp/base-1335-base-test BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test

Fixes #1335
Part of #562