fix(changelog): make CHANGELOG_OLD.md rotation more resilient

[Copilot]

When rotating old changelog entries from README.md into CHANGELOG_OLD.md, the Older entries are in [CHANGELOG_OLD.md](CHANGELOG_OLD.md). footer line was being included in the last rotated entry and moved into CHANGELOG_OLD.md, leaving the README without a footer.

Root cause

parseChangelogFile extends each ### version entry to the next ##/### header. When the footer appears after the last version entry (no subsequent header follows), it's absorbed into that entry's content. On rotation, the entry—footer included—moves to CHANGELOG_OLD.md.

Fix

• packages/plugin-changelog/src/index.ts — In executeCheckStage, after parsing README entries and only when CHANGELOG_OLD.md is present, detect the footer pattern at the end of the last parsed entry. If found, strip it from the entry and prepend it to parsed.after, so it flows into changelog_after (written back to README only, never to CHANGELOG_OLD.md).

• packages/plugin-changelog/src/index.test.ts — Two new tests: one verifying the footer is moved to changelog_after during the check stage, one verifying post-rotation file contents (footer in README, absent from CHANGELOG_OLD.md).

README.md ## Changelog
  ### 2.0.0 ...        ← stays in README
  ### 1.0.0 ...        ← rotated to CHANGELOG_OLD (no footer)
  Older entries are in [CHANGELOG_OLD.md]...  ← stays in README ✓