Add full-flow order tracing and per-section selector discovery

[Multipixelone]

Add a trace-order CLI command and an opt-in live trace that dump a
rendered DOM, a selector probe, and a screenshot at every checkpoint of
the buy flow — product page, cart, cart view, delivery, payment, and the
review page. Unlike dump-dom/verify-selectors (which stop at the
product page), this reaches the checkout/review surface where the
place-order, order-total, and payment selectors finally render, so they
become discoverable and verifiable.

Implementation threads an optional FlowTracer through the real
BasePurchaser.buy() and the store-specific _start_checkout(), defaulting
to a shared no-op singleton. A normal worker/orchestrator buy pays only one
inert call per checkpoint — the live order path is unchanged. The CLI
command hard-forces DRY_RUN (like dry-run), so it always halts at the
review page before Place Order. Every checkpoint is best-effort: a capture
failure is logged and swallowed and can never alter the order's outcome.

• purchase.py: FlowTracer/TraceStep/_NullTracer/new_run_id; checkpoints
  in buy() and both _start_checkout overrides; trace full-page shot prefix.
• cli.py: trace-order command with a per-step PASS/MISS digest.
• config.py: trace_orders flag (ROOMIEORDER_TRACE_ORDERS, default off) wired
  into the orchestrator for opt-in live tracing.
• New /trace-order skill; bring-up/triage-failure/AGENTS docs updated.
• Tests for the dry-run contract, step output, no-op default, checkpoint
  ordering, and best-effort probe-failure isolation.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01F5GyaYcKJEkPz7nCaKfrm7