app: allow starting in subscribe mode with a loader and no static config

[methakon]

Problem

SubscribeRunE rejects a configuration with no subscriptions and no inputs:

numInputs := len(a.Config.Inputs)
if len(subCfg) == 0 && numInputs == 0 {
    return errors.New("no subscriptions or inputs configuration found")
}

A clustered instance does not have to carry any static configuration. The loader is the source of
targets and subscriptions, and startLoader is already written for exactly that case - it no-ops
when no loader is set, waits for the instance to become leader, and then polls.

The problem is ordering. This check runs before startLoader is reached, so an instance whose entire
purpose is to have its targets managed through the API or a KV store cannot start at all. The error
names subscriptions and inputs as the only sources, when a loader is a third one.

Change

The loader is taken into account, so this is rejected only when there is genuinely nothing configured
to collect from:

if len(subCfg) == 0 && numInputs == 0 && len(a.Config.Loader) == 0 {

An empty loader map still counts as no loader, so a config with a blank loader: section behaves as
it did before.

Test

TestSubscribeRequiresSubscriptionsInputsOrLoader covers the four meaningful combinations - loader
only, loader plus inputs, inputs only, and neither - and pins that an empty loader map does not
count as a loader. That last case is the one that could otherwise turn a typo'd
loader: {} into a silent no-op.

A note on the test's scope

The validation sits inside a cobra command and is reached through a chain of GetX() config
readers, so the test exercises the predicate against a real config.Config rather than driving
SubscribeRunE end to end. That keeps it a unit test, but it does mean the test and the production
condition are two expressions of the same rule rather than one calling the other. If a reviewer would
prefer the condition extracted into a small helper that both the command and the test call, that is a
reasonable alternative and I am happy to do it that way instead - the current shape keeps the diff to
one line plus a test.

Verification

Command	Result
go test ./pkg/app/... -count=1	ok, 1.2s
go test ./pkg/app/ -run TestSubscribeRequires -count=1	5 cases pass
go build ./...	clean
go vet ./pkg/app/	clean
gofmt -l on both files	clean

Fixes #787