fix(changelog): make independent prepends union-mergeable

[seonghobae]

Summary

• configure the changelog for union merging
• add structural checks for non-empty sections and balanced fences
• preserve the current changelog shape while reducing prepend conflicts

Closes #1961

Verification

• python -m pytest -q tests/test_changelog_structure_contract.py (2 passed)
• python -m compileall -q tests/test_changelog_structure_contract.py
• git diff --check

[coderabbitai]

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Review configuration:

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Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 7f168662-5efe-4969-815c-c3e6c1e58235

📥 Commits

Reviewing files that changed from the base of the PR and between cb0872c and d148e04.

📒 Files selected for processing (3)

• .gitattributes
• CHANGELOG.d/20260911-changelog-union-contract.md
• tests/test_changelog_structure_contract.py

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[opencode-agent]

Pull request overview

OpenCode could not approve from deterministic current-head evidence because GitHub Checks have failed.

Findings

1. HIGH Current-head GitHub Checks - Fix failed required checks before approval

• Problem: Failed same-head checks remain for d148e04664cc9de92d56e2bc0a95c9cd94e35b92.
• Root cause: The model-unavailable evidence fallback is allowed only when peer GitHub Checks are complete and clean.
• Fix: Read and fix the failed check logs below, then rerun the current-head checks.
• Regression test: Keep the model-unavailable fallback gated on an empty failed-check rollup.

Failed checks:

• Required Noema Review/noema-review: FAILURE (https://github.com/ContextualWisdomLab/.github/actions/runs/34563771019/job/103151931588)
• noema-review check run: failure (https://github.com/ContextualWisdomLab/.github/actions/runs/34563771019/job/103151931588)

Changed-File Evidence Map

flowchart LR
  PR["PR changed files"] --> Evidence["OpenCode bounded evidence"]
  Evidence --> S1["Repository file: .gitattributes"]
  S1 --> I1["repository behavior"]
  I1 --> R1["Review risk: Repository file: .gitattributes"]
  R1 --> V1["required checks"]
  Evidence --> S2["Repository file: 20260911-changelog-union-contract.md"]
  S2 --> I2["repository behavior"]
  I2 --> R2["Review risk: Repository file: 20260911-changelog-union-contract.md"]
  R2 --> V2["required checks"]
  Evidence --> S3["Test: test_changelog_structure_contract.py"]
  S3 --> I3["regression suite"]
  I3 --> R3["Review risk: Test: test_changelog_structure_contract.py"]
  R3 --> V3["targeted test run"]

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[opencode-agent]

OpenCode Review Overview

• Head SHA: d148e04664cc9de92d56e2bc0a95c9cd94e35b92
• Workflow run: 34564755362
• Workflow attempt: 1
• Gate result: REQUEST_CHANGES
• Coverage gate: success
• Model pool: exhausted
• Verdict: REQUEST_CHANGES
• Formal review: fix(changelog): make independent prepends union-mergeable #2091 (review)

[seonghobae]

HIGH/functional finding on exact head d148e04664cc9de92d56e2bc0a95c9cd94e35b92: the merge-driver rule does not cover the files this PR is adding. .gitattributes contains only CHANGELOG.md merge=union, while the new release fragments live under CHANGELOG.d/*.md. In the PR worktree, git check-attr merge -- CHANGELOG.d/20260911-changelog-union-contract.md returns merge: unspecified. The focused contract passes (2 passed), but it only asserts the existing root CHANGELOG.md rule and does not exercise the fragment path. If the intended contract is independent changelog fragments, add a pattern/rule covering CHANGELOG.d/ (and a test using an actual fragment path) or narrow the PR scope and clarify that fragments are not protected.