Implement PVC-backed Git workspace cache (design-pvc-workspace.md)

Add pkg/workspace.Manager that provides Acquire/Release semantics for Git
repos. When GIT_CACHE_DIR is set and a GitRepository has spec.cache.enabled,
controllers use on-disk bare clones with incremental fetch; otherwise they
fall back to the existing in-memory clone behavior.

Key changes:
- New pkg/workspace package with Manager, Workspace, GC, Deepen, and
  context-based injection (WithManager/GetManager)
- GitRepository API gains spec.cache (CacheConfig) and status.cache
  (CacheStatus) with deepcopy and CRD schema updates
- Push, sync, and resolver reconcilers now use workspace.Acquire/Release
  instead of inline memory.NewStorage + git.CloneContext
- Shallow clone by default; sync/resolver deepen as needed for merge-base
- New Prometheus metrics: workspace_acquire_duration_seconds,
  workspace_cache_hit_total, workspace_cache_miss_total
- Deployment manifests include commented-out PVC configuration
- Comprehensive unit tests for workspace package (21 tests)
- E2E tests for cache-free and cache-enabled push flows

https://claude.ai/code/session_01NULGxaCLPMT1yDVEAsffc5

cmd/push-controller/main.go

@@ -1,16 +1,26 @@
 package main
 
 import (
+	"os"
+
 	"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"
+	"github.com/imjasonh/git-k8s/pkg/workspace"
 )
 
 func main() {
 	ctx := signals.NewContext()
 	go health.ServeMetrics(ctx, ":9090") //nolint:errcheck
+
+	// GIT_CACHE_DIR enables PVC-backed workspace caching when set.
+	// Push controller uses shallow clones since it only needs to push refs.
+	cacheDir := os.Getenv("GIT_CACHE_DIR")
+	wsMgr := workspace.NewManager(cacheDir, true /* shallow */)
+	ctx = workspace.WithManager(ctx, wsMgr)
+
 	sharedmain.MainWithContext(ctx, "push-controller", push.NewController)
 }