docs: add reproducible debugging guidance

[ded-furby]

Summary

• Add a Base-specific reproducible debugging workflow to docs/testing.md.
• Document completion checks that require fresh validation evidence before merge.

Issue

Closes #500

Validation

• git diff --check

Demo Impact

None. Documentation/process only.

Notes

Documentation-only change. Maintainer review is requesting a couple of small doc corrections before merge.

[codeforester]

Thanks for the contribution. This is directionally useful and the docs-only scope is right, but please fix two small documentation details before we merge:

1. In docs/testing.md, the runtime ownership bullet points to lib/shell/runtime/, which does not exist. Please change that to lib/bash/runtime/ for the runtime shell code, and keep lib/shell/ only for shell startup/profile snippets if you mention both.
2. Please make the diagnostic guidance explicit that basectl check and basectl doctor are read-only/non-mutating commands. That is part of #500's acceptance criteria and helps keep the debugging workflow clear.

After those are updated, git diff --check is enough validation for this docs-only PR.