Add full support for diffing OpenAPI 3.1 documents including:

Document level:
- WebhooksDiff for OpenAPI 3.1 webhooks
- JSONSchemaDialectDiff for JSON Schema dialect specification

Info object:
- SummaryDiff for Info.Summary field

License object:
- IdentifierDiff for SPDX license identifiers

Schema object (JSON Schema 2020-12):
- ConstDiff for const keyword
- ExamplesDiff for examples array
- PrefixItemsDiff for tuple validation
- ContainsDiff, MinContainsDiff, MaxContainsDiff for array containment
- PatternPropertiesDiff for pattern-based properties
- DependentSchemasDiff for conditional schemas
- PropertyNamesDiff for property name validation
- UnevaluatedItemsDiff, UnevaluatedPropertiesDiff

Closes #52

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add comprehensive tests for webhooks and new OpenAPI 3.1 / JSON Schema 2020-12 fields:
- Webhook add/delete/modify tests
- JSONSchemaDialect diff tests
- Info.Summary diff tests
- License.Identifier diff tests
- Schema fields: const, examples, prefixItems, contains, minContains,
  maxContains, patternProperties, dependentSchemas, propertyNames,
  unevaluatedItems, unevaluatedProperties

Coverage improvements:
- webhooks_diff.go: 87.5-100%
- schema_diff.go: 86.4-100%
- info_diff.go: 81.2-100%
- license_diff.go: 83.3-94.1%

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Add additional tests for webhook diff functionality:
- Test Empty() with populated but empty diff struct
- Test webhooks only in base spec (deletion case)
- Test webhooks only in revision spec (addition case)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

OpenAPI 3.1 changed exclusiveMinimum/exclusiveMaximum from boolean
modifiers to numeric bound values. Updated oasdiff to work with the
new ExclusiveBound type from kin-openapi that supports both formats.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

- Add score property with exclusive bounds to Order schema in test data
- Add test for ExclusiveMinDiff/ExclusiveMaxDiff with numeric values
- Fix exclusiveBoundToValue to extract actual values from ExclusiveBound struct

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

The docker/metadata-action automatically converts slashes to dashes,
so use its output for feature branch builds to avoid invalid tag errors.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Update replace directive to oasdiff/kin-openapi@1da4078 which includes:
- JSON Schema 2020-12 identity keywords ($id, $anchor, $dynamicRef, $dynamicAnchor)
- Content vocabulary (contentMediaType, contentEncoding, contentSchema)
- Discriminator support for anyOf
- jsonSchemaDialect validation
- mutualTLS security scheme
- License url/identifier mutual exclusivity
- Paths optional in 3.1
- JSON Schema 2020-12 validation in openapi3filter

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Support diffing if/then/else, dependentRequired, $id, $anchor,
$dynamicRef, $dynamicAnchor, contentMediaType, contentEncoding,
and contentSchema fields added in kin-openapi's OpenAPI 3.1 support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>