docs: add debugging tips to DEVELOPMENT.md

Mirrors the equivalent improvements on notification-controller (#362),
helm-controller (#462), and image-reflector-controller (#245):
RUNTIME_NAMESPACE, --concurrent=1 (plus --concurrent-ssa=1), flux
suspend, and cross-controller setup with source-controller for users
reproducing kustomize-controller issues locally.

Signed-off-by: alliasgher <alliasgher123@gmail.com>

Author: alliasgher

DEVELOPMENT.md

@@ -91,4 +91,53 @@ Deploy `kustomize-controller` into the cluster that is configured in the local k
 make deploy
 ```
 
-Running the above will also deploy `source-controller` and its CRDs to the cluster.
+Running the above will also deploy `source-controller` and its CRDs to the cluster.
+
+## Debugging the controller locally
+
+When reproducing an issue or stepping through reconciliation logic, the
+following knobs make local runs cheaper and the resulting logs easier to
+read.
+
+### Limit the watched namespace
+
+The controller watches every namespace by default. To narrow it to a single
+namespace, set the `RUNTIME_NAMESPACE` environment variable before invoking
+`make run`:
+
+```sh
+RUNTIME_NAMESPACE=flux-system make run
+```
+
+### Reduce reconcile concurrency
+
+Each `Kustomization` reconcile is processed concurrently (default
+`--concurrent=4`, plus an independent `--concurrent-ssa=4` for server-side
+apply). When debugging it is almost always easier to follow a serial
+trace; pass `--concurrent=1 --concurrent-ssa=1` so reconciles and SSA
+operations run one at a time:
+
+```sh
+go run ./main.go --concurrent=1 --concurrent-ssa=1
+```
+
+### Suspend unrelated objects
+
+If the controller is sharing a cluster with other Flux objects, suspend
+anything not relevant to the test you're running so their reconciles don't
+interleave with yours:
+
+```sh
+flux suspend kustomization <name>
+```
+
+Resume with `flux resume kustomization <name>` when you're done.
+
+### Cross-controller interactions
+
+`kustomize-controller` consumes artifacts produced by `source-controller`
+(`GitRepository`, `OCIRepository`, `Bucket`). When testing a path that
+needs a fresh artifact, run `source-controller` locally too (or keep its
+in-cluster instance running) and point both at the same management
+cluster. Suspending unrelated controllers' objects per the previous step
+avoids racing reconciles.