Clear stale bootstrap temp refs instead of failing on resume mismatch

When a temp ref from a previous run doesn't match any planned checkpoint (e.g., the user changed --target-max-pack-bytes between runs, or a previous subdivide created midpoints that no longer appear in the new plan), bootstrap now deletes the stale temp ref and starts the branch fresh rather than failing with "does not match any planned checkpoint".

Temp refs are an internal implementation detail of batched bootstrap, not user-visible state. Treating a mismatch as fatal forced users to manually delete refs on the target before retrying — unnecessary friction for a routine parameter change.