feat: atomization pipeline with job queue, atomizer client, WS hub, and board UI (phase 7)

- internal/extsvc: circuit breaker (§11.16) + retrying bearer JSON client
- atomizer client: §5.1 contract incl. defensive coefficient normalization
  and extension coefficient range (0,2]
- persisted jobs collection: atomic claim, panic recovery, exponential
  backoff (max 3), stale-running requeue, shutdown-safe bookkeeping
- subdivide (async 202, atomizing status, re-run replaces unpublished
  children after confirm) and extend (sibling task, source budget)
- failure path reverts status and notifies the consultant on final attempt
- PATCH task editing with bounty recompute, budget cascade to descendants
- publish single + bulk; task detail endpoint role-scoped
- coder/websocket hub: multiplexed channels, origin check, 30s heartbeats;
  client ws.js with reconnect + 15s polling fallback
- consultant atomization board: tree by root, coefficient sliders with live
  per-parent sum indicator, bounty preview, modals, shimmer, atomizer-down
  gating via /api/v1/service-health
- fix: tasks external-ref unique index is partial, not sparse (compound
  sparse indexed every task through customerId and broke child inserts)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/extsvc/breaker_test.go

@@ -0,0 +1,75 @@
+package extsvc
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestBreakerLifecycle(t *testing.T) {
+	b := NewBreaker()
+	now := time.Date(2026, 6, 12, 12, 0, 0, 0, time.UTC)
+	b.now = func() time.Time { return now }
+	boom := errors.New("boom")
+
+	if b.State() != "closed" || !b.Allow() {
+		t.Fatal("fresh breaker must be closed")
+	}
+
+	// two failures: still closed
+	b.Record(boom)
+	b.Record(boom)
+	if b.State() != "closed" || !b.Allow() {
+		t.Fatal("breaker must stay closed below threshold")
+	}
+
+	// third consecutive failure opens it
+	b.Record(boom)
+	if b.State() != "open" {
+		t.Fatalf("state = %s, want open", b.State())
+	}
+	if b.Allow() {
+		t.Fatal("open breaker must reject calls")
+	}
+
+	// after the cooldown one probe is allowed (half-open)
+	now = now.Add(61 * time.Second)
+	if !b.Allow() {
+		t.Fatal("half-open must allow one probe")
+	}
+	if b.Allow() {
+		t.Fatal("only one probe may be in flight")
+	}
+	if b.State() != "half-open" {
+		t.Fatalf("state = %s, want half-open", b.State())
+	}
+
+	// failed probe re-opens for another cooldown
+	b.Record(boom)
+	if b.Allow() {
+		t.Fatal("failed probe must re-open the breaker")
+	}
+	now = now.Add(61 * time.Second)
+	if !b.Allow() {
+		t.Fatal("next probe window")
+	}
+
+	// successful probe closes it fully
+	b.Record(nil)
+	if b.State() != "closed" || !b.Allow() || !b.Allow() {
+		t.Fatal("successful probe must close the breaker")
+	}
+}
+
+func TestBreakerSuccessResetsCount(t *testing.T) {
+	b := NewBreaker()
+	boom := errors.New("x")
+	b.Record(boom)
+	b.Record(boom)
+	b.Record(nil) // non-consecutive failures never open
+	b.Record(boom)
+	b.Record(boom)
+	if b.State() != "closed" {
+		t.Fatal("interleaved success must reset the failure count")
+	}
+}