feat(executor): budget-gate the dispatcher — reroute to local or block (Phase 6)

The pool now consults a BudgetGate before taking an agent slot. If running on
the chosen paid provider could breach its rolling window (estimated by the
task's max_budget_usd), it reroutes to the free local runner when one is
registered; otherwise it stops before running and marks the task
BUDGET_EXCEEDED. serve.go builds a budget.Accountant from [budget] config and
wires it into the pool (only when limits are configured).

Allows the QUEUED → BUDGET_EXCEEDED transition, since the gate blocks a queued
task before it ever reaches RUNNING. Covered by pool tests for both the block
and reroute paths against a fake gate.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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