feat(guardrails): enforce guardrails in workflow entry points

[mikehostetler]

Summary

• add mix jido_shell.guardrails as a first-class namespace/layout validation task
• introduce guardrail rules for legacy path detection, required session module layout, and namespace prefix consistency
• enforce guardrails in scripted workflows via:
  • mix quality alias
  • CI guardrails job
• document contributor guardrail expectations
• add dedicated tests for rule behavior and mix-task failure surfacing

Validation

• mix test test/jido/shell/guardrails_test.exs test/mix/tasks/jido_shell.guardrails_test.exs --seed 0
• mix test --seed 0
• mix quality
• mix jido_shell.guardrails

Closes #15

[mikehostetler]

Superseded by #19.

main moved (layout migration + guardrail refactor), which left this branch in a dirty merge state. The replacement PR lands the remaining guardrail workflow/extension work cleanly on top of current main.