backport: add self-review step to instructions

[TomasTomecek]

Summary

• Adds a self-review checklist (5 criteria) as the final step before the backport agent reports success, covering patch correctness, spec file integrity, no unrelated changes, SRPM existence, and patch naming
• Extracts shared self-review criteria into _self_review.j2 to avoid duplication between y-stream and z-stream templates
• Criteria 1, 2a/2b, and 5 are self-guarding for zero-patch scenarios (spec-only changes)
• Criterion 2b accounts for %autosetup/%autopatch specs that don't need explicit %patch directives
• Criterion 2c (pre-existing Patch tags intact) always applies, even on the spec-only path
• Y-stream Criterion 3 qualifies the Release field exception to exclude the spec-only path

Test plan

• Both templates render identically to pre-dedup output (verified via rendered diff)
• pytest ymir/agents/tests/unit/test_jinja2_templates.py — 31 tests pass

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces a reusable self-review template (_self_review.j2) for backport instructions, integrating it into both standard and z-stream backport instruction templates. The review feedback suggests quoting the SRPM path placeholder in the shell command to prevent word-splitting, and utilizing Jinja2 block assignments ({% set %} ... {% endset %}) instead of escaped multi-line strings to improve template readability and maintainability.

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[gemini-code-assist]

Quoting the file path placeholder in the shell command is a safer practice to prevent potential word-splitting or syntax errors if the generated SRPM path contains spaces or special characters.

   `test -f "<path-to-srpm>" && echo exists || echo missing`

[gemini-code-assist]

Using Jinja2's block assignment ({% set var %} ... {% endset %}) is the idiomatic and much more readable way to define multi-line strings in templates. It avoids manual \n escape sequences and improves maintainability.

{% set spec_only_note %}
   Note: If this was a spec-only change (step 3d path), no patch files are
   generated — criteria 1, 2a, 2b, and 5 will not apply. Criterion 2c still
   applies: verify that pre-existing Patch tags were not accidentally modified.
{% endset %}
{% set criterion_3_body %}
   From the git diff output, verify the `%changelog` section was not modified.
   (Changing the Release field is acceptable in y-stream, unless this was a
   spec-only change via step 3d — in that case the Release field must not be
   modified either.)
{% endset %}

[gemini-code-assist]

Using Jinja2's block assignment ({% set var %} ... {% endset %}) is the idiomatic and much more readable way to define multi-line strings in templates. It avoids manual \n escape sequences and improves maintainability.

{% set criterion_3_body %}
   From the git diff output, verify all of the following:
   a. The `%changelog` section was not modified.
   b. Your changes (visible in the diff) did not modify the `Release:` field.
{% endset %}