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

1 files changed, 1 insertions, 0 deletions

diff --git a/internal/executor/executor.go b/internal/executor/executor.go
index 6d6c528..4333f51 100644
--- a/internal/executor/executor.go
+++ b/internal/executor/executor.go
@@ -1025,6 +1025,7 @@ func (p *Pool) execute(ctx context.Context, t *task.Task) {
 		TaskID:    t.ID,
 		StartTime: time.Now().UTC(),
 		Status:    "RUNNING",
+		Agent:     agentType,
 	}
 
 	// Pre-populate log paths so they're available in the DB immediately —