docs: add debugging tips to DEVELOPMENT.md

[alliasgher]

Summary

Adds a 'Debugging the controller locally' section to DEVELOPMENT.md covering knobs not already in 'How to run the controller locally':

• RUNTIME_NAMESPACE to scope the watch
• --concurrent=1 to serialize reconciles for a clean trace
• flux suspend for unrelated objects sharing the cluster

Cross-controller setup is already documented (or n/a for this controller) and is not duplicated here.

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

[alliasgher]

Self-correcting this before it wastes a reviewer's time: two of the tips in this PR are wrong, and I only found out because @stefanprodan reviewed the same text on the kustomize-controller version (fluxcd/kustomize-controller#1646).

The RUNTIME_NAMESPACE tip does not work. That variable is only read when --watch-all-namespaces is false:

if !watchOptions.AllNamespaces {
    watchNamespace = os.Getenv("RUNTIME_NAMESPACE")
}

and the flag defaults to true (fluxcd/pkg/runtime/controller/watch.go), so RUNTIME_NAMESPACE=flux-system make run on its own changes nothing. I verified this controller has the identical code. Stefan also made the broader point that narrowing the watch would hide cross-namespace behaviour you often need while debugging, so the section should go rather than be corrected.

The --concurrent=1 tip is backwards as general debugging advice: goroutine leaks, deadlocks and races only surface under concurrency, and it is redundant anyway once unrelated objects are suspended.

That is most of the substance, so I do not think there is a useful PR left here. Happy to close it, or to cut it down to just the accurate remainder if a maintainer thinks that is worth keeping. Sorry for the noise, and for sending the same wrong text to several repos at once.

[alliasgher]

Closing this. Two of the three tips were wrong rather than merely imprecise, and once they are removed there is not enough left to justify a review cycle.

For the record, so this is searchable if anyone tries the same thing later:

• Limit the watched namespace was wrong. RUNTIME_NAMESPACE is only read when --watch-all-namespaces is false, and that flag defaults to true, so the documented command had no effect. @stefanprodan also made the stronger point that narrowing the watch would hide cross-namespace behaviour you often need while debugging, so the section should not exist even in a corrected form.
• Reduce reconcile concurrency was backwards as general advice: goroutine leaks, deadlocks and races only surface under concurrency, and it is redundant once unrelated objects are suspended.
• Suspend unrelated objects was the only sound tip, and it amounts to mentioning that flux suspend <kind> --all exists, which is not worth a PR on its own.

Thanks @stefanprodan for the review that caught this, and apologies to the other repos for receiving the same wrong text. If a maintainer does want a debugging section in DEVELOPMENT.md, I am happy to write one from scratch against what the flags actually do.