chore: remove git-cliff and automated changelog generation

Removed git-cliff configuration and workflow as requested.
The changelog will now be maintained manually.

- Deleted .github/workflows/changelog.yml
- Deleted .github/cliff.toml
- Removed git-cliff attribution from CHANGELOG.md

This simplifies the repository and removes unnecessary automation.

Author: lainra

.github/cliff.toml

@@ -42,6 +42,3 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Revert
 
 - Use peter-evans/create-pull-request action ([16816c6](https://github.com/lainra/claude-code-telemetry/commit/16816c662854b63f8dc42719a11081bf07af4a32))
-
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-Generated by [git-cliff](https://github.com/orhun/git-cliff)