feat(guardrails): enforce workflows and formalize extension path

[mikehostetler]

Summary

• add explicit CI guardrails job to enforce namespace/layout checks in workflow entry points
• extend Jido.Shell.Guardrails with pluggable rule support (default_rules/0, configured extension rules, explicit rules: option)
• split built-in checks into rule modules for safer convention evolution
• add GUARDRAILS.md and wire it into docs/package extras
• update contributor/agent workflow docs with guardrail expectations and convention-change checklist
• expand mix-task coverage (including failure path via --root) and add extension-rule tests

Validation

• mix test test/jido/shell/guardrails_test.exs test/jido/shell/guardrails_extension_test.exs test/mix/tasks/jido_shell.guardrails_test.exs --seed 0
• mix test --seed 0
• mix quality
• mix coveralls (reported total coverage: 73.2% on current mainline baseline)

Closes #15
Closes #16