Fix cligen schema walker to recurse through nested array items

The request/response field-tree walker only unwrapped one array level
before checking whether the element schema was an object, so a nested
array field like an OR-of-AND filter tree (array<array<object>>) never
expanded its item fields the way a plain array<object> field did — and
its rendered type label collapsed to the uninformative "array<array>".
Both --help and the skill-card generator read from this same walker, so
every filters/source_filters/target_filters field across silence,
escalate, inhibit, and unsubscribe rules (and any other nested-array
field) silently dropped its key/oper/vals schema.

Generalize the array case to unwrap items through any depth of nested
arrays (bounded, not per-command special-cased) before testing for an
object leaf, and apply the same fix to the type-label formatter. Add
unit tests covering the nested-object, nested-scalar, and single-level
regression cases.

Also rewrite the channel card's maintenance-silence example, which
built a rule with only a time window — servers reject that as having no
matching scope — and add a general rule for constructing `filters` from
an incident's own labels (severity + distinguishing labels, with the
value/key exclusions and key vocabulary needed to avoid silently
building a rule that never matches).

Author: ysyneu