Add the agenthub-dev skill and the changelog details directory

What changed

Added .agents/skills/agenthub-dev/SKILL.md, fixing the model-support development workflow: sync official docs into llmsdk_docs/, capture a live streaming tool-call exchange with thinking into the git-ignored api_captures/, implement paired Python/TypeScript protocol clients with bijective message conversion, and verify with model-scoped e2e tests only.
The workflow makes four situations hard stops that require asking the user: unclear or unfetchable official docs, a missing provider API key, any live API request error, and non-obvious UniConfig key mappings.
Added api_captures/ to .gitignore as the home for raw API captures; where docs and captures disagree, the capture wins.
Added the changelog/ directory: each CHANGELOG.md entry keeps one brief line and links to a detail file here (see changelog/README.md).