Plan file should be cleared after successful implementation

When using plan mode, the plan file keeps the old plan around even after it's been fully implemented. It just sits there taking up context tokens until the next plan happens to overwrite it.

Current behavior:
Plan written → Approved → Implemented → Old plan stays in file → Eventually overwritten by next plan

Expected behavior:
Plan written → Approved → Implemented → Plan file cleared

Why isn't the plan file cleaned up after implementation? Seems like unnecessary context waste, and in long sessions it can be confusing if the stale plan gets picked up after context compaction.