feat(api,web): budget headroom endpoint + UI chips (Phase 6)

Adds GET /api/budget returning per-provider rolling-window headroom (empty when
budget gating is unconfigured), wired from serve.go via SetBudget. The web UI
polls it and renders a chip per limited provider in the header, flagging any
under 20% remaining. New pure JS helpers formatBudgetHeadroom/
renderBudgetHeadroom are unit-tested; the endpoint is covered by Go handler
tests (empty/reports/500). UI render not browser-verified in this environment.

Completes Phase 6: spend accounting + dispatcher gating + observability surface.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

1 files changed, 36 insertions, 0 deletions

diff --git a/internal/api/budget.go b/internal/api/budget.go
new file mode 100644
index 0000000..eee5d09
--- /dev/null
+++ b/internal/api/budget.go
@@ -0,0 +1,36 @@
+package api
+
+import (
+	"net/http"
+
+	"github.com/thepeterstone/claudomator/internal/budget"
+)
+
+// budgetReporter exposes per-provider spend headroom. Satisfied by
+// *budget.Accountant; an interface keeps the handler testable.
+type budgetReporter interface {
+	All() ([]budget.Headroom, error)
+}
+
+// SetBudget wires the budget accountant so GET /api/budget can report headroom.
+func (s *Server) SetBudget(b budgetReporter) {
+	s.budget = b
+}
+
+// handleGetBudget returns per-provider rolling-window spend headroom. When no
+// budget is configured it returns an empty list so the UI simply shows nothing.
+func (s *Server) handleGetBudget(w http.ResponseWriter, _ *http.Request) {
+	if s.budget == nil {
+		writeJSON(w, http.StatusOK, []budget.Headroom{})
+		return
+	}
+	hs, err := s.budget.All()
+	if err != nil {
+		writeJSON(w, http.StatusInternalServerError, map[string]string{"error": err.Error()})
+		return
+	}
+	if hs == nil {
+		hs = []budget.Headroom{}
+	}
+	writeJSON(w, http.StatusOK, hs)
+}