Allow [Hidden] on namespace classes; add Help/Schema properties

[Mpdreamz]

[Hidden] can now be placed on a namespace handler class to suppress the entire namespace from the help listing. The attribute gains two named properties — Help (default true) and Schema (default false) — giving callers explicit control over whether an item is hidden from users, from tooling, or from both.

Prompt summary: Extend [Hidden] to accept AttributeTargets.Class so a namespace handler type can opt its namespace out of the public --help output. The use-case is internal or automation namespaces (such as a release namespace) that CI scripts should still be able to invoke, but that should not clutter the help listing end users see. As part of this, add Help and Schema named properties so callers can control each surface independently.

Why

Namespaces registered via AddNamespace<T> have no way to suppress themselves from the root or parent help listing short of not registering them at all. Internal automation namespaces end up visible to every end user running --help, even when they are never meant for direct use.

What

Attribute surface

[Hidden] now allows AttributeTargets.Class and exposes two named properties: Help (default true) and Schema (default false). Plain [Hidden] hides an item from the help listing but leaves it fully visible in __schema output. Setting Schema = true is wired through the generator pipeline but has no effect on namespace __schema output yet — see below.

Generator pipeline

A new GetHiddenFlags helper replaces HasHiddenAttribute everywhere in the generator. It returns a (HideHelp, HideSchema) pair by reading the named properties from the attribute. Both flags flow as IsHidden (help gate) and IsHiddenInSchema (schema gate) on CommandModel, ParameterModel, RegistryNode.NamedCommandNamespaceChild, ChildNamespaceSnapshot, and AIMapNamespace.

Help output

The root and namespace help printers now filter child namespaces where IsHidden is true. A hidden namespace is still fully callable — the dispatch router routes to it normally. Only the listing in the parent's --help output is suppressed.

Schema output

The hidden: true flag on commands and parameters now requires an explicit [Hidden(Schema = true)] rather than being emitted by every bare [Hidden]. Existing test fixtures that relied on [Hidden] producing hidden: true in schema are updated to [Hidden(Schema = true)]. The hidden property is NOT yet emitted on namespace objects: the upstream cli-schema meta-schema doesn't define it for namespaces, so adding it caused schema-conformance failures. This is tracked at cli-schema/cli-schema#7.

Verify

dotnet test tests/Nullean.Argh.IntegrationTests/ -c Release --filter "FullyQualifiedName~Hidden"
# RootHelp_does_not_list_hidden_namespaces — namespace with [Hidden] absent from help output
# Hidden_namespace_is_still_callable — hidden namespace still dispatches correctly
# Schema_hidden_namespace_appears_in_schema_without_hidden_flag — [Hidden] alone leaves schema clean

Breaking: [Hidden] on a command method or parameter no longer emits hidden: true in __schema output by default. Use [Hidden(Schema = true)] to restore that behaviour.

Out of scope: hidden on namespace __schema objects is deferred pending the upstream meta-schema change at cli-schema/cli-schema#7. [MiddlewareAttribute<T>] on a class (namespace-level middleware) is also a natural companion but requires non-trivial generator changes and will be addressed separately.

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