docs: add change note

docs/PROJECT-CHANGE-NOTE.md

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+# Project Change Note
+
+Open Problem Lab should preserve important maintainership changes in a lightweight, searchable format.
+
+Use this note for changes that affect review gates, evidence standards, generated outputs, contributor workflow, or problem-pack scope.
+
+## Entry format
+
+```markdown
+## PCN-YYYY-NNN — Short name
+
+**Date:** YYYY-MM-DD.
+
+**Change:** One sentence.
+
+**Context:** Why the change was needed.
+
+**Options considered:** Short bullet list.
+
+**Chosen path:** What changed.
+
+**Expected benefit:** What should improve.
+
+**Known cost:** What becomes harder or slower.
+
+**Review trigger:** What would make the project revisit this change.
+```
+
+## When to add an entry
+
+Add an entry when a change affects how contributors work or how accepted artifacts are interpreted.
+
+Do not add entries for routine typo fixes, small copy edits, or ordinary issue triage.
+
+## Reviewer guidance
+
+Reviewers should ask for a project change note when a PR changes a policy but the reason would otherwise live only in a PR thread.
+
+## Acceptance test
+
+A future maintainer should be able to understand what changed, why, and what would justify changing it again.