Add Prometheus metrics, health probes, Git timeouts, and unit tests

Critical production-readiness improvements:

- Prometheus metrics: reconcile count/latency per controller, Git operation
  duration (clone/push/ls-remote) via pkg/metrics with /metrics on :9090
- Health probes: leverage Knative's built-in health server on :8080, add
  readiness/liveness probes to all four deployment manifests
- Git operation timeouts: 5-minute context deadline on all git.CloneContext
  and PushContext calls to prevent indefinite blocking on large repos
- Unit tests: new ReconcileKind-level tests for push (pending→failed on
  missing repo, auth error paths), sync (branch-not-found, LastSyncTime),
  resolver (missing repo, partial hashes, empty phase), and repowatcher
  (ls-remote errors, empty remote, unchanged branches)

https://claude.ai/code/session_01PaXbaSqhVEqj97kpY4v6rt

cmd/push-controller/main.go

@@ -2,10 +2,15 @@ package main
 
 import (
 	"knative.dev/pkg/injection/sharedmain"
+	"knative.dev/pkg/signals"
 
+	"github.com/imjasonh/git-k8s/pkg/health"
+	_ "github.com/imjasonh/git-k8s/pkg/metrics" // register Prometheus metrics
 	"github.com/imjasonh/git-k8s/pkg/reconciler/push"
 )
 
 func main() {
-	sharedmain.Main("push-controller", push.NewController)
+	ctx := signals.NewContext()
+	go health.ServeMetrics(ctx, ":9090") //nolint:errcheck
+	sharedmain.MainWithContext(ctx, "push-controller", push.NewController)
 }