docs: add the 0.2.1 changelog entry
Adds the 0.2.1 entry to CHANGELOG.md. pyproject.toml and stackone_adk/__init__.py are already at 0.2.1 on main.
0.2.1 needs to reach PyPI before stackone-ai 3.0.0 does (stackone-ai-python#200). The published 0.2.0 depends on stackone-ai[mcp]>=2.8.0 with no upper bound, and stackone_adk/__init__.py imports SearchConfig and SearchMode, which 3.0 removes. Once 3.0 is out, a fresh pip install stackone-adk==0.2.0 on Python 3.11+ resolves stackone-ai 3.0 and fails on import.
After merge: rm -rf dist/ && uv build && uv publish, then tag v0.2.1, following the existing v0.2.0 tag.
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Summary by cubic
Adds the 0.2.1 entry to CHANGELOG.md, covering the stackone-ai[mcp] cap and the plugin description update. This lets 0.2.1 ship to PyPI before stackone-ai 3.0 removes SearchConfig and SearchMode, which would otherwise break fresh installs of 0.2.0.