Add explanatory base-demo commands

[codeforester]

Summary

• Add env and manifest commands to the Base demo manifest.
• Expand activation state with non-secret reference-demo metadata.
• Update the walkthrough, validator, and BATS tests to exercise the new commands.

Validation

• ./tests/validate.sh
• bats tests/demo_test.bats
• git diff --check
• /Users/rameshhp/work/base/bin/basectl demo base-demo --workspace /Users/rameshhp/work/base-demo-worktrees -- --non-interactive

Closes #10