Add optional task runner wrappers

[codeforester]

Summary

• Add optional justfile and Taskfile.yml wrappers for the common Base loop.
• Keep every wrapper as a thin delegation to basectl for check, CI check, test, build, demo, and service status.
• Document the wrappers as optional and add contract/validation guards so they do not become a second command contract.

Fixes #180.

Validation

• ./tests/validate.sh
• git diff --check