feat(guardrails): document and test convention extension path

[mikehostetler]

Summary

• add GUARDRAILS.md with explicit extension workflow for future namespace/layout convention changes
• support configured extension rules via config :jido_shell, :guardrail_rules, [...]
• include guardrails guide in package/docs extras
• add extension-focused tests for:
  • configured rule injection
  • targeted rule execution without configured extensions
  • invalid extension rule configuration errors

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

Depends on #17
Refs #16

[mikehostetler]

Superseded by #19.

This stacked branch is now stale after upstream main changes. The replacement PR applies the intended guardrail extension/docs/test work directly on current main.