Add prepare-release codex skill

Introduce .codex/skills/prepare-release/SKILL.md documenting a prepare-release workflow for serious_python. The file lists required inputs and step-by-step actions to bump package versions (pubspecs, podspec, Android build.gradle), run pub get for example apps, and add changelog entries from git history while excluding trivial/chore/duplicate items.

Author: FeodorFitsner

.codex/skills/prepare-release/SKILL.md

@@ -0,0 +1,20 @@
+---
+name: prepare-release
+description: Use when asked to prepare new serious_python release by bumping versions and author release notes.
+---
+
+## Inputs
+
+* Previous serious_python version from repo tags.
+* Whether it's minor or major release.
+
+## Steps
+
+* Take latest serious_python release version from the repo and
+  increment third (patch) digit to get the next version if it's a minor release
+  or second (minor) digit if it's a major release.
+* Set new version in pubspec.yaml of all packages in /src.
+* Set new s.version in src/serious_python_darwin/darwin/serious_python_darwin.podspec.
+* Set new version in src/serious_python_android/android/build.gradle.
+* Run pub get for all apps in src/serious_python/example to refresh their pubspec.lock with new version.
+* Add a new entry into all packages' CHANGELOG.md from a git log since the last release. Do not add chore/trivial/duplicate items, add items with related issue or PR.