Restructure upstream-merge prompt flow and add explicit documentation-impact gate

[Copilot]

The upstream merge prompt’s detailed sections had drifted from the workflow outline, making execution order ambiguous. It also lacked a strong guard against recurring misses where user-facing upstream ports land without corresponding documentation updates.

• Workflow structure alignment

  • Reordered and renumbered the prompt so the Workflow Overview and detailed sections map directly.
  • Established a single contiguous sequence (Step 1 … Step 11) and removed mismatched/ambiguous step groupings.

• Documentation follow-through enforcement

  • Strengthened Step 9 guidance so documentation review is a required checkpoint before moving to finalization.
  • Added explicit fallback guidance: when user-facing changes do not require doc edits, PR body must include a concise rationale under a clear heading (e.g., Documentation Impact).

• Checklist hardening

  • Updated the final checklist to require explicit explanation when docs are intentionally unchanged for user-facing upstream changes.
  • Kept scope limited to the upstream-merge prompt file; no runtime SDK behavior changes.

## Step 9: Update Documentation
Review and complete this documentation checklist before proceeding to Step 10.
If you determine no docs changes are needed, document that decision and rationale in the PR body under a clear heading (for example, `Documentation Impact`).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Prompt agentic-merge-upstream.prompt.md is not well structured</issue_title>
<issue_description>The workflow defines steps, but then the details of each step are not properly ordered / numbered based on the workflow outline.

More importantly, it is common that an agentic merge from upstream adds new capabilities but then forgets to update documentation. This has been a recurring problem.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Prompt agentic-merge-upstream.prompt.md is not well structured #167

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