Execute Plan Action Route

Execute one record-change plan action (link_client / update_contact).

Re-verifies payload.from against the live record (409 with the current value on a mismatch, nothing written), performs the change via the same underlying writes the main pipeline agent's tools use, emits deal.plan_action_executed, removes the action from the plan, and logs the note that enqueues the replan. Returns {"removed": true}.