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, 11 insertions, 0 deletions

diff --git a/internal/config/config.go b/internal/config/config.go
index 94f3ec7..58de95c 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -69,6 +69,17 @@ type Config struct {
 	VAPIDEmail       string        `toml:"vapid_email"`
 	ClaudeConfigDir  string        `toml:"claude_config_dir"`
 	LocalModel       LocalModel    `toml:"local_model"`
+	Budget           BudgetConfig  `toml:"budget"`
+}
+
+// BudgetConfig configures rolling per-provider spend caps. With no providers
+// set, budget gating is disabled (nothing is blocked) — there is intentionally
+// no default cap; the user must opt in by configuring limits.
+type BudgetConfig struct {
+	// Window is the rolling lookback duration (e.g. "5h"); defaults to 5h.
+	Window string `toml:"window"`
+	// Provider5hUSD maps a provider (claude/gemini) to its USD cap within Window.
+	Provider5hUSD map[string]float64 `toml:"provider_5h_usd"`
 }
 
 func Default() (*Config, error) {