fix: preserve canonical worktree reap reasons (#4284)

[atomantic]

Summary

• Remove the legacy short-reason remap so shared canonical worktree-* ownership codes pass through.
• Update reaper expectations to the canonical reason values.
• Add the changelog fragment.

Closes #4284

Test plan

• cd server && NODE_ENV=test npm test -- worktreeReap
• cd server && NODE_ENV=test npm test -- worktreeReap worktreeOwnership worktreeManager
• git diff --check