Clarify instruction source for stop guidance

[Copilot]

Summary

Clarify that the stop-on-failure guidance is defined by the environment’s
developer instructions, not by repository files.

Objectives:

• State where the guidance is defined

• Avoid implying AGENTS.md contains developer instructions

• Instruction source

  • Document that the guidance comes from system/developer instructions

Issue resolution:

• N/A

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major
  version

Please fill out either the "Small Change" or "Big Change" section (the latter
includes the numbered subsections), and delete the other.

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a
  single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

Big Change

• To merge, I will use "Create a merge commit". That is, this change is large
  enough to require multiple units of work (i.e., it should be multiple commits).

1. Does this do what we want it to do?

Required:

• Product Management: I have confirmed with the stakeholders that the
  objectives above are correct and complete.
• Testing: I have added or modified at least one "min-case" configuration
  file to test this change. Every objective above is represented in at least one
  cfg.
• Testing: I have considered likely and/or severe edge cases and have
  included them in testing.

If applicable:

• Testing: this pull request introduces an important feature or bug fix that
  we must test often. I have updated the weekly-test configuration files, not
  just a "min-case" one.
• Testing: this pull request adds at least one new possible parameter to the
  cfg. I have tested using this parameter with and without any other parameter
  that may interact with it.

2. Are the implementation details accurate & efficient?

Required:

• Logic: I have visually inspected the entire pull request myself.
• Logic: I have left GitHub comments highlighting important pieces of code
  logic. I have had these code blocks reviewed by at least one other team member.

If applicable:

• Dependencies: This pull request introduces a new dependency. I have
  discussed this requirement with at least one other team member. The dependency
  is noted in zppy/conda, not just an import statement.

3. Is this well documented?

Required:

• Documentation: by looking at the docs, a new user could easily understand
  the functionality introduced by this pull request.

4. Is this code clean?

Required:

• Readability: The code is as simple as possible and well-commented, such
  that a new team member could understand what's happening.
• Pre-commit checks: All the pre-commits checks have passed.

If applicable:

• Software architecture: I have discussed relevant trade-offs in design
  decisions with at least one other team member. It is unlikely that this pull
  request will increase tech debt.

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