Add CHANGELOG for the 1.0.0 release

Document the openai 1.x migration, gpt-4o/gpt-4o-mini support and new
default, Python 3.12 upgrade, dependency bumps, Docker improvements
and the text-davinci-003 removal.

CHANGELOG.md

@@ -0,0 +1,35 @@
+# Changelog
+
+All notable changes to this project are documented in this file.
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [1.0.0]
+
+### Added
+- `gpt-4o` and `gpt-4o-mini` models, with `gpt-4o-mini` as the new default.
+- `VERSION` file and this changelog.
+
+### Changed
+- Migrated from the deprecated `openai==0.28` API to the `openai>=1.x` SDK
+  (`AsyncOpenAI` client, new chat/image/audio/moderation methods, updated
+  error classes).
+- Default model is now `gpt-4o-mini` instead of the outdated `gpt-3.5-turbo`
+  (new users, `/new`, and the `ChatGPT` fallback).
+- Upgraded the Docker base image to Python 3.12 (3.8 is end-of-life).
+- Bumped dependencies: `python-telegram-bot` 20.8, `pymongo` 4.6.3,
+  `PyYAML` 6.0.2, `python-dotenv` 1.0.1, `tiktoken` >= 0.7.0.
+- Docker: smaller image (`pip --no-cache-dir`), unbuffered logs
+  (`PYTHONUNBUFFERED`), live-mounted config, expanded `.dockerignore`,
+  removed the obsolete Compose `version` key.
+
+### Removed
+- `text-davinci-003` and its legacy completion code paths (the model was
+  shut down by OpenAI).
+
+### Fixed
+- Corrected `gpt-4o` pricing and scores in `models.yml`.
+- `tiktoken` now recognizes the `o200k_base` encoding used by `gpt-4o`.
+- Removed debug logging that fired on every incoming message.
+- Narrowed bare `except:` clauses so shutdown/cancellation propagates.
+- Stopped tracking `.DS_Store`; various typo and docs fixes.