[ci] Auto-resolve predictable PR conflicts

[coisa]

Related Issue

Closes #192

Motivation / Context

• PR branches have repeatedly become unmergeable only because main moved a workflow-managed .github/wiki pointer or because CHANGELOG.md gained adjacent Unreleased entries.
• The changelog resolver treats the current base branch changelog as authoritative and replays branch-only Unreleased entries into the active Unreleased section, so release promotion on main does not cause new branch notes to land under the wrong published version.

Changes

• Add a reusable auto-resolve-conflicts.yml workflow plus packaged consumer wrapper.
• Add a local github/resolve-predictable-conflicts action that scans same-repository PRs, attempts a local merge, and only pushes a resolution when conflicts are limited to .github/wiki and/or CHANGELOG.md.
• Add UnreleasedChangelogConflictResolver with tests covering normal drift, release-moved base state, and deduplication.
• Document the predictable-conflict workflow in GitHub Actions and branch-protection docs.
• Include small Rector-required cleanup in existing config tests so the refactor gate stays green.

Verification

• composer dev-tools (timed out after 300s during metrics; before timeout it reported full PHPUnit success: 519 tests, 1600 assertions, and only the Rector findings that are now fixed)
• composer dev-tools refactor -- --json
• composer dev-tools code-style -- --json
• composer dev-tools phpdoc -- --json
• composer dev-tools docs -- --json
• composer dev-tools changelog:check
• composer dev-tools tests -- --filter=Changelog
• composer dev-tools tests -- --filter='Changelog|ComposerDependencyAnalyserConfigTest|ECSConfigTest'
• php -l .github/actions/github/resolve-predictable-conflicts/resolve-changelog.php && bash -n .github/actions/github/resolve-predictable-conflicts/run.sh
• YAML parsed with Ruby YAML.load_file for the new workflow/action files
• Manual changelog-conflict simulation confirmed branch-only entries stay in the current Unreleased after base release promotion

Documentation / Generated Output

• README updated
• docs/ updated
• Generated or synchronized output reviewed

Changelog

• Added a notable CHANGELOG.md entry

Reviewer Notes

• The workflow remains intentionally conservative: fork PRs are skipped, already-mergeable PRs are skipped, and any conflict outside .github/wiki/CHANGELOG.md aborts back to manual resolution.
• .github/wiki keeps the PR side because it represents the preview branch pointer for that PR.

Additional Review Validation

• Synthetic local Git/GitHub harness exercised .github/actions/github/resolve-predictable-conflicts/run.sh end to end by:
  • creating a temporary repository with a real CHANGELOG.md merge conflict;
  • mocking gh pr view, gh workflow view, and gh workflow run;
  • rewriting the action clone to the temporary bare repository;
  • confirming the action pushed a resolution commit;
  • confirming the resolved changelog kept the branch-only entry in current Unreleased and kept the base release entry under 1.2.0;
  • confirming tests.yml dispatch was requested with publish-required-statuses=true.

[github-actions]

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• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-237/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-237/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-237/metrics/