Expand the Base demo walkthrough

[codeforester]

Summary

• Add workspace discovery, project diagnostics, and command listing to the walkthrough.
• Pass explicit workspace paths to supported Base commands so worktree-based runs resolve the intended project.
• Update BATS coverage for the expanded non-interactive flow.

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 #6