feat(webhook): trigger brain-sync on Gitea push instead of 15-min poll

Adds POST /webhooks/brain-sync: verifies Gitea's HMAC-SHA256 signature,
checks the push is to mathias/brain main, then creates a one-off Job from
the existing brain-sync CronJob's template (same script the 15-min poll
already runs, just triggered on-demand). Off by default -- opt in via
GITEA_WEBHOOK_SECRET, since it needs Job-create RBAC in the "brain"
namespace a fresh deploy won't have.

10 new tests (internal/webhook), including a fake-clientset reactor to
simulate server-side GenerateName expansion, which the plain fake tracker
doesn't do on its own.

Needs (follow-up, infra repo): RBAC granting ingestion's ServiceAccount
get on cronjobs/brain-sync + create on jobs in the brain namespace, the
GITEA_WEBHOOK_SECRET env, and the actual Gitea webhook registration.

ingestion/cmd/server/main.go

@@ -34,8 +34,27 @@ import (
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/vectorstore"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/watcher"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/webhook"
+	"k8s.io/client-go/kubernetes"
+	"k8s.io/client-go/rest"
+	"k8s.io/client-go/tools/clientcmd"
 )
 
+// kubeClient builds an in-cluster Kubernetes client (falls back to
+// $KUBECONFIG for local dev/testing against a real cluster).
+func kubeClient() (kubernetes.Interface, error) {
+	if cfg, err := rest.InClusterConfig(); err == nil {
+		return kubernetes.NewForConfig(cfg)
+	}
+	rules := clientcmd.NewDefaultClientConfigLoadingRules()
+	cc := clientcmd.NewNonInteractiveDeferredLoadingClientConfig(rules, &clientcmd.ConfigOverrides{})
+	cfg, err := cc.ClientConfig()
+	if err != nil {
+		return nil, fmt.Errorf("kube config (no in-cluster, no kubeconfig): %w", err)
+	}
+	return kubernetes.NewForConfig(cfg)
+}
+
 // claudeSink converts each claudewatcher.Batch into a raw session dump
 // under brain/archive/claude-sessions/<host>/. Deliberately NOT a wiki
 // note (api.WriteNote / brain/wiki/) — raw full transcripts out-ranked
@@ -352,6 +371,29 @@ func main() {
 		logger.Info("claudewatcher started",
 			"sessions_dir", claudeDir, "host", host, "interval", interval)
 	}
+
+	// Gitea push webhook -> on-demand brain-sync Job, instead of waiting up
+	// to 15 minutes for the next CronJob poll. Off by default (opt in via
+	// GITEA_WEBHOOK_SECRET) since it needs Job-create RBAC in the "brain"
+	// namespace that a fresh deploy won't have granted yet.
+	var webhookHandler *webhook.Handler
+	if webhookSecret := os.Getenv("GITEA_WEBHOOK_SECRET"); webhookSecret != "" {
+		kc, err := kubeClient()
+		if err != nil {
+			logger.Error("brain-sync webhook: kube client", "err", err)
+			os.Exit(1)
+		}
+		webhookHandler = &webhook.Handler{
+			Secret:      webhookSecret,
+			Clientset:   kc,
+			Namespace:   envOr("BRAIN_SYNC_NAMESPACE", "brain"),
+			CronJobName: envOr("BRAIN_SYNC_CRONJOB", "brain-sync"),
+			WatchRepo:   envOr("BRAIN_SYNC_WATCH_REPO", "mathias/brain"),
+			Logger:      logger,
+		}
+		logger.Info("brain-sync webhook enabled", "namespace", webhookHandler.Namespace, "cronjob", webhookHandler.CronJobName)
+	}
+
 	if vectorStore != nil {
 		embedSyncInterval := envInt("BRAIN_EMBED_SYNC_INTERVAL", 300)
 		vectorstore.StartSync(ctx, brainDir, vectorStore,
@@ -373,6 +415,9 @@ func main() {
 	mux.HandleFunc("POST /promote", h.Promote)
 	mux.HandleFunc("POST /backfill-embeddings", h.BackfillEmbeddings)
 	mux.HandleFunc("GET /pass-rate", h.PassRate)
+	if webhookHandler != nil {
+		mux.Handle("POST /webhooks/brain-sync", webhookHandler)
+	}
 	jwtValidator, err := chassisauth.NewJWTValidator(ctx, os.Getenv("DEX_ISSUER_URL"), os.Getenv("MCP_AUDIENCE"))
 	if err != nil {
 		logger.Error("build jwt validator", "err", err)