Drop unused execution_id column from kv_store table

Background

With the KV delta table change (duroxide 0.1.26), the execution_id column in the kv_store table is no longer read by any code path. KV pruning is now instance-scoped (not execution-scoped), and the column serves no purpose.

Current state

The column was intentionally kept in the 0020 migration for backward compatibility — existing deployments have the column populated, and dropping it during a rolling upgrade could cause issues with old nodes that still write to it.

Action

In a future release (after all deployments are on the kv_delta version):

1. Create a new migration that:
   • ALTER TABLE kv_store DROP COLUMN execution_id;
   • DROP INDEX IF EXISTS idx_kv_store_execution;
2. Update any stored procedures that still reference execution_id in the kv_store INSERT path

Priority

Low — the column is harmless, just wastes a few bytes per row.

Start by reading the 0020 migration and inspecting the current kv_store schema, idx_kv_store_execution index, and stored procedures that handle kv_store inserts. Create the future migration to remove the column and index, update procedures that still reference execution_id, and verify the migration is compatible with the stated kv_delta rollout stage.