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, 38 insertions, 0 deletions

diff --git a/internal/storage/db_test.go b/internal/storage/db_test.go
index 0e67e02..4d744a4 100644
--- a/internal/storage/db_test.go
+++ b/internal/storage/db_test.go
@@ -309,6 +309,44 @@ func TestListTasks_WithLimit(t *testing.T) {
 	}
 }
 
+func TestSpendByProviderSince(t *testing.T) {
+	db := testDB(t)
+	now := time.Now().UTC()
+
+	tk := &task.Task{
+		ID: "spend-task", Name: "S", Agent: task.AgentConfig{Type: "claude", Instructions: "x"},
+		Priority: task.PriorityNormal, Retry: task.RetryConfig{MaxAttempts: 1, Backoff: "linear"},
+		Tags: []string{}, DependsOn: []string{}, State: task.StatePending, CreatedAt: now, UpdatedAt: now,
+	}
+	if err := db.CreateTask(tk); err != nil {
+		t.Fatal(err)
+	}
+
+	// Two recent claude execs, one recent gemini, one old claude (outside window).
+	execs := []*Execution{
+		{ID: "s1", TaskID: "spend-task", StartTime: now.Add(-1 * time.Hour), Status: "COMPLETED", CostUSD: 1.5, Agent: "claude"},
+		{ID: "s2", TaskID: "spend-task", StartTime: now.Add(-2 * time.Hour), Status: "COMPLETED", CostUSD: 2.0, Agent: "claude"},
+		{ID: "s3", TaskID: "spend-task", StartTime: now.Add(-30 * time.Minute), Status: "COMPLETED", CostUSD: 0.75, Agent: "gemini"},
+		{ID: "s4", TaskID: "spend-task", StartTime: now.Add(-10 * time.Hour), Status: "COMPLETED", CostUSD: 99, Agent: "claude"},
+	}
+	for _, e := range execs {
+		if err := db.CreateExecution(e); err != nil {
+			t.Fatalf("create exec %s: %v", e.ID, err)
+		}
+	}
+
+	spends, err := db.SpendByProviderSince(now.Add(-5 * time.Hour))
+	if err != nil {
+		t.Fatalf("SpendByProviderSince: %v", err)
+	}
+	if got := spends["claude"]; got != 3.5 {
+		t.Errorf("claude spend: want 3.5 (old $99 excluded), got %v", got)
+	}
+	if got := spends["gemini"]; got != 0.75 {
+		t.Errorf("gemini spend: want 0.75, got %v", got)
+	}
+}
+
 func TestCreateExecution_AndGet(t *testing.T) {
 	db := testDB(t)
 	now := time.Now().UTC().Truncate(time.Second)