Gate the expanded Base demo workflow in CI

[codeforester]

Summary

• Add Base-backed repository baseline validation to CI with basectl repo check.
• Exercise command listing and the hello command through basectl run.
• Use explicit workspace paths for CI test and demo commands.
• Correct README output for basectl run outside an activated shell.

Validation

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

Closes #5