Add repo-watcher-controller for polling remote Git refs

Introduces a fourth controller that watches GitRepository resources and
polls their remotes via git ls-remote on a configurable per-repo interval
(spec.pollInterval, defaulting to 30s). It auto-creates, updates, and
deletes GitBranch CRDs to reflect the actual state of the remote,
enabling fast detection of external pushes to trigger downstream sync.

Key changes:
- Add PollInterval field to GitRepositorySpec
- New pkg/reconciler/repowatcher with controller and reconciler
- New cmd/repo-watcher-controller binary
- CRD manifest updated for pollInterval
- Deployment manifest for the new controller
- Unit tests for ref filtering, naming, and poll interval logic
- CLAUDE.md updated with new controller docs and Go module workaround

https://claude.ai/code/session_01P7xfBqARU5DFS8QJ4uDPVj

cmd/repo-watcher-controller/main.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"knative.dev/pkg/injection/sharedmain"
+
+	"github.com/imjasonh/git-k8s/pkg/reconciler/repowatcher"
+)
+
+func main() {
+	sharedmain.Main("repo-watcher-controller", repowatcher.NewController)
+}