bggpipe init: guided, idempotent first-run setup

One command replaces the clone-era checklist: creates photos/ and
data/, writes a default config.toml, prompts for the four credentials
with hidden input (appended to a 0600 .env, only the missing ones,
values never echoed), and offers the one-time Chromium download.
Re-runs report status and fill gaps; without a TTY it reports instead
of hanging. Groundwork for any future publishing path — PyPI or a
bundled app both need exactly this wizard.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

CLAUDE.md

@@ -19,7 +19,7 @@ Full design lives in `bgg-shelf-pipeline-spec.md` (read it before changing pipel
 
 ## Commands
 
-- `uv sync` — install deps (Python 3.12+, managed by **uv**; use `uv add`, never pip). Playwright needs a one-time `uv run playwright install chromium`.
+- `uv sync` — install deps (Python 3.12+, managed by **uv**; use `uv add`, never pip). `uv run bggpipe init` handles first-run setup (folders, .env credentials, the one-time `playwright install chromium`).
 - `uv run bggpipe <stage>` — run a pipeline stage. Non-secret settings come from `config.toml` (username, dirs, vision model, rate limit); `--config` overrides the path.
 - `uv run pytest` — the suite runs fully offline against fixtures. Tests marked `live` hit the real BGG API (read-only) and are skipped unless you pass `--run-live`.
 - `uv run ruff check` / `uv run ruff format` — lint (rules E, F, I, UP, B, SIM) and format.