Adds a flag on tasks to distinguish manual completions from the cascade auto-completion performed by complete_subtask_and_check_parent. Partial index on (done_at, auto_completed) supports the Phase 8 daily-count query without bloating the tasks index footprint. Forward-only: pre-migration completed rows default to 0 (they count). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>