feat(budget,storage,config): per-provider spend accounting substrate (Phase 6)

Adds the budget accountant foundation:
- storage: a per-execution `agent` column (additive migration, populated by the
  dispatcher) and SpendByProviderSince(since), summing cost_usd per provider in
  a window. Accurate attribution survives a task running on different providers
  across retries.
- internal/budget: Accountant over a SpendSource + Limits, exposing Headroom
  (remaining/fraction per provider), Allow (would estCost breach the cap), and
  All (one query, sorted). Unconfigured/local providers are unlimited.
- config: a [budget] section (window + provider_5h_usd map). No default cap —
  gating is opt-in by configuring limits.

Fully unit-tested; dispatcher gating and the API/UI surface follow.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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