v0.2.0 — kubectl-style configuration; keychain removed

- Unified ~/.config/eocube/eocube.yml (authentication/storage/stac/mcp/context)
- Global --context; `eocube setup` (init + auth/storage/stac/context add)
- OS keychain removed (tokens persist in the config file; re-login once)
- pydantic added as a runtime dependency
- Per-context storage modeled as providers + schemes (behavior to follow)
- Docker image stamps the package version from the tag

Library import API is backwards compatible; CLI surfaces changed.