docs: add changelog covering all released tags

CHANGELOG.md

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+# Changelog
+
+All notable changes to PureQL.CSharp.Model.OpenAPI.Schema are documented here.
+
+Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+
+---
+
+## [0.1.0-preview.0.2.2] — 2026-08-21
+
+### Fixed
+
+- `PureQLQueryDocumentTransformer` no longer mistakes a grouping node in the
+  PureQL JSON Schema for a schema of its own. Group names (e.g.
+  `booleanOperations`, holding `and`/`or`/`not`) share a namespace with JSON
+  Schema keywords, so a group whose only conflicting child was named `not`
+  had its members silently dropped and left the `$ref`s pointing at them
+  dangling.
+- `PureQLQueryDocumentTransformer` no longer throws `NullReferenceException`
+  when transforming a document that has no generated schemas yet.
+
+## [0.1.0-preview.0.2.1] — 2026-08-21
+
+- Maintenance release: dependency and build updates.
+
+## [0.1.0-preview.0.2.0] — 2026-08-13
+
+- Maintenance release: dependency and build updates.
+
+## [0.1.0-preview.0.1.3] — 2026-08-12
+
+- Maintenance release: dependency and build updates.
+
+## [0.1.0-preview.0.1.2] — 2026-08-06
+
+### Fixed
+
+- Pinned `Microsoft.OpenApi` to 2.11.0, fixing a vulnerability
+  (GHSA-v5pm-xwqc-g5wc / NU1903) reachable through the lower version that
+  `Microsoft.AspNetCore.OpenApi` resolves by default.
+
+## [0.1.0-preview.0.1.1] — 2026-06-10
+
+- Maintenance release: dependency and build updates.
+
+## [0.1.0-preview.0.1.0] — 2026-05-13
+
+### Added
+
+- Initial release: `PureQLQueryDocumentTransformer`, an
+  `IOpenApiDocumentTransformer` that replaces the auto-generated `Query`
+  schema in ASP.NET Core OpenAPI documents with the canonical PureQL JSON
+  Schema, so API consumers see the correct query type schema.
+- Targets .NET 10.0.