Add yaml struct tags to Schema for bidirectional YAML (un)marshaling

[alecrubin]

This revives #171 with the context that was missing the first time. My follow-up there only went up after the PR had already been closed, so the use case never really got a hearing. Putting it here up front.

What this does

Mirrors the existing json struct tags on every Schema field with matching yaml tags. Purely additive: no behavioral change for JSON users, the tags are inert unless a YAML (un)marshaler is actually used.

Why a JSON→YAML converter doesn't cover it

The suggestion on #171 was to use a JSON→YAML converter instead. That only solves the write path. The use case here is bidirectional:

• Read path. We persist schemas as hand-edited YAML files and yaml.Unmarshal them back into *jsonschema.Schema. With no yaml tags, the decoder falls back to the lowercased field name, which matches none of the $-prefixed or camelCase keys — $schema, $defs, prefixItems, additionalProperties, patternProperties, etc. A converter can't help on decode; it needs tags on the library's own struct (the only alternative is a parallel mirror struct, which is far more friction than the tags it would replace).
• Order preservation. Properties is a *orderedmap.OrderedMap, and property order is structurally meaningful in JSON Schema. Round-tripping through a generic map[string]any converter flattens it through an unordered map and reorders fields; marshaling straight from the struct keeps order intact.

On the "duplicating tags adds friction" concern

It's ~50 tags that mechanically match the existing json names, added once, with zero ongoing cost — they're ignored entirely by anyone who never imports a YAML package. The friction is a one-time mirror; the capability it unlocks (round-tripping schemas through YAML without losing field order or $-prefixed keys) isn't otherwise reachable without forking the struct.