### Added

- MkDocs Material configuration for the documentation site.
- CI docs build job and release docs gate.
- Release workflow deployment to GitHub Pages via `mkdocs gh-deploy`.
- PyPI-backed GHCR server image publishing for `ghcr.io/lpalbou/abstractgateway-server`.
- CI validation build for the local server Docker image recipe.
- Docker server image, Compose profile, and deployment documentation.
- `docs`, `server`, `vision`, and `multimodal` optional dependency extras.
- Discovery metadata for AbstractCore capability plugins (`voice`, `audio`, `vision`, and future `music`).

### Changed

- Version metadata aligned across `pyproject.toml`, package `__version__`, and FastAPI app metadata.
- The server install profile now mirrors the newer AbstractRuntime/Core multimodal stack: `AbstractRuntime[multimodal]>=0.4.6`, `abstractcore[remote,media,tools,tokens,compression,vision,voice,audio]>=2.13.10`, `abstractvision>=0.3.1`, and `abstractvoice>=0.9.0`.
- The server Docker/Compose profile now documents workflow-backed image generation through AbstractVision, direct Gateway TTS/STT through AbstractVoice, and provider-dependent prompt-cache controls.
- Gateway voice/audio endpoints now accept AbstractVoice's newer local/remote backend environment knobs in addition to the existing Gateway-scoped settings.

### Notes

- Release scope is intentionally explicit: TTS and STT have direct Gateway endpoints; generated images are available through Runtime/Core workflows with AbstractVision installed and configured, but Gateway does not yet expose a direct image-generation HTTP endpoint.
- Prompt-cache support is provider-level control-plane support. This release does not add a Gateway-owned CachedSession lifecycle API.
- `flows/bundles/article@dev.flow` was inspected and left untracked. It is a local `dev` bundle generated by the Gateway publisher, not a release artifact.
