Add a lightweight Python CLI demo

[codeforester]

Summary

• Add a tiny standard-library Python CLI under lib/python/base_demo_cli.
• Expose it through the manifest as the python-info command using the Base project environment.
• Validate the command in README, demo walkthrough, BATS tests, and CI.

Validation

• ./tests/validate.sh
• bats tests/demo_test.bats
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/tests.yml")'
• git diff --check
• /Users/rameshhp/work/base/bin/basectl run base-demo --workspace /Users/rameshhp/work/base-demo-worktrees python-info
• /Users/rameshhp/work/base/bin/basectl demo base-demo --workspace /Users/rameshhp/work/base-demo-worktrees -- --non-interactive

Closes #11