[P] Pre-execution budget enforcement for autonomous agents — the concurrency problem with in-process counters

Most agent cost controls share the same failure mode: they observe or report spend after execution. Under concurrent agents sharing a budget, the window between "check balance" and "commit usage" lets multiple agents both proceed when only one should. The fix requires atomic pre-execution reservation: lock estimated exposure before the action, commit actual after, release on failure. The semantics are identical to a two-phase commit.