docs: add verified getting-started workflow

[jemsbhai]

Summary

• add a Docker-first getting-started guide with clone, startup, first-use, lifecycle, troubleshooting, source-development, and production-boundary instructions
• add a checked-in Next.js development environment template and surface the quick start near the top of the README
• harden the documented Compose path with a writable Next.js cache tmpfs and bounded MinIO initialization retries
• add documentation, configuration, and deployment contract coverage so the copy-paste workflow cannot drift silently

Why

CorpusKit had all required Dockerfiles and locked dependency manifests, but startup guidance was scattered and the native-development sequence was not runnable as written. A fresh empty-volume smoke also exposed a transient MinIO DNS race, and the root environment example contained blank typed settings that could not be loaded directly.

User impact

A new user now needs only Git and a running Docker Engine with Compose v2 for the local demo. Host Python, Node.js, npm, and eSpeak are not required. The guide clearly labels the demo identity as local-only and documents persistence, safe shutdown, destructive reset, port overrides, and the separate production path.

No requirements.txt was added: pyproject.toml + uv.lock remain the authoritative Python requirements, and package.json + package-lock.json remain authoritative for the web app.

Validation

• fresh isolated Compose builds and empty-volume --profile web up --build --detach --wait
• API readiness, authenticated demo session, real corpus creation, and browser rendering with no console errors
• docker compose --profile web config --quiet
• docker compose --profile '*' config --quiet
• 184 focused documentation/deployment/release/config tests passed
• 188 broader deployment/release/config tests passed, 56 opt-in tests skipped
• Ruff format/check, mypy (133 source files), Prettier, uv lock --check, and git diff --check passed