Generate or update the project changelog.

Arguments (optional): $ARGUMENTS

Instructions:
1. Parse arguments: first argument is optional date (YYYY-MM-DD), remaining text is semicolon-separated description of changes.
   - If no date provided, use today's date.
   - If no description provided, summarize changes from the current conversation or use "Aktualizacja projektu".
2. Detect changed files using `git status --porcelain`.
3. Read the existing changelog file `changelog/<date>.md` if it exists.
4. Merge new entries with existing ones (deduplicate both "Co zrobiono" points and file list).
5. Write/update the file with this structure:

```
# YYYY-MM-DD

## Co zrobiono

- point 1
- point 2

## Zmienione pliki

- `path/to/file`
```

6. Show the user what was written.

Rules:
- Never overwrite existing entries — only append and deduplicate.
- File list should include both git-detected changes AND files touched in the current conversation.
- Use Polish section headers as shown above.
- Keep descriptions concise but informative.