Blocked: task requires Orchestra code changes (migration 020 to add last_commit_sha column to task_runs, sync.py push_main to return main_sha, and process_merge_candidate to update task_runs after merge). Orchestra /home/ubuntu/Orchestra is read-only in this sandbox environment, and the Orchestra DB (/data/orchestra/orchestra.db) is not mounted. Artifacts created in SciDEX worktree: (1) spec at docs/planning/specs/ecead4e7-d198-4fd2-9bd1-299abc4de6a3_populate_task_runs_last_commit_sha_spec.md documenting all required Orchestra changes; (2) scripts/backfill_task_runs_last_commit_sha.py - idempotent backfill script (safe to run, checks column existence, exits gracefully if not yet migrated). The Orchestra-side changes must be applied via a separate PR to the Orchestra repo. Soft-depends on Orchestra PR #223.