docs: add debugging tips to DEVELOPMENT.md

Adds a 'Debugging the controller locally' section covering knobs that
are not already in 'How to run the controller locally':

- RUNTIME_NAMESPACE to scope the watch
- --concurrent=1 --concurrent-ssa=1 to serialize reconciles and
  server-side apply for a clean trace
- flux suspend kustomization for unrelated objects sharing the cluster

Cross-controller setup with source-controller (port-forward +
SOURCE_CONTROLLER_LOCALHOST) is already documented under 'How to run
the controller locally' so it is not duplicated here.

Assisted-by: Claude/claude-opus-4-7

Author: alliasgher

DEVELOPMENT.md

@@ -91,4 +91,44 @@ 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.