Build base-demo interactive walkthrough

[codeforester]

Summary

Build the interactive walkthrough inside the base-demo repository.

Scope

• Add the main demo script, probably demo/demo.sh or base-demo.sh.
• Support interactive mode with pauses and --non-interactive mode for CI.
• Keep the script narrative-driven and beginner-friendly.
• Stop on errors with clear recovery guidance.
• Avoid long-running installs, external accounts, or heavyweight dependencies.

Suggested Demo Steps

• Base-managed project structure.
• base_manifest.yaml contracts.
• Brewfile delegation with lightweight tools such as jq and curl.
• Project virtualenv behavior.
• Project activation via activate.source.
• basectl run for declared commands.
• basectl test for the test contract.
• basectl demo base-demo recursively demonstrating the new capability.

Acceptance Criteria

• The demo is pleasant to run interactively.
• --non-interactive runs without blocking.
• Tests validate each meaningful demo step.
• The README shows the expected clone, setup, and demo flow.

Source design: docs/base-demo-spec.md.