`awa dev` sandbox: one-command local demo environment (candidate)

## Motivation

Onboarding candidate, explicitly P3: a single command that runs a disposable Postgres (docker), migrates, starts a demo worker plus the UI with seeded jobs — so a first-time evaluator sees the dashboard populated within a minute of install, before writing any code.

## Scope (only if cheap)

- `awa dev` — starts a container-backed Postgres (or uses `--database-url` if given), runs `awa migrate`, launches a built-in demo worker (a few illustrative kinds: fast, slow-with-progress, flaky-with-retries, one cron), seeds a few hundred jobs, opens `awa serve`.
- Everything torn down on exit; nothing installed beyond the existing binary; clearly labelled "not for production".
- Do **not** embed Postgres in the binary (ADR-001 stays: Postgres is the dependency).

## Decision

Build only if the implementation stays a thin orchestration layer over existing commands. If it grows real complexity, close as not-planned — the quickstarts already serve this need adequately.

## References

Roadmap: `docs/0.7-roadmap.md` (WS-6, NEW-17, P3 candidate).

Start with docs/0.7-roadmap.md for WS-6 and NEW-17, then inspect the existing `awa migrate`, `awa serve`, and quickstart flows. Confirm whether a thin `awa dev` orchestration layer can use Docker-backed Postgres, seed demo jobs, launch the worker and UI, and tear everything down; otherwise close it as not planned.

Written by the indexing model from the issue text.