add changelog

Author: hugodutka

CHANGELOG.md

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+# Changelog
+
+## v0.4.0
+
+### Breaking changes
+
+- If you're running agentapi behind a reverse proxy, you'll now likely need to set the `--allowed-hosts` flag. See the [README](./README.md) for more details.
+
+### New features
+
+- Sourcegraph Amp support
+- Added a new `--allowed-hosts` flag to the `server` command.
+
+### Fixes
+
+- Updated Codex support after its TUI has been updated in a recent version.