docs: document pull and add the baseline export + zero-diff demo step

[Kiran01bm]

Document pg-sprite pull and add a baseline-export + zero-diff verification step to the demo tour.

Why

The pull CLI landed without a user-facing doc, so the intended onboarding loop — export a live schema, then prove diff against the export is empty — existed only in the PR that added the command. Users adopting pg-sprite on an existing database need that loop spelled out and demonstrated, not inferred from --help.

What

• docs/pull.md: what pull enumerates, per-table results (not fail-fast), the refusal set, and the export → diff --json zero-statement verification loop; linked from README.md and docs/README.md.
• docs/cli-output-examples.md: an executed pull example.
• demo/tour.sh: run_pull runs pull, asserts the file count, then asserts diff --json returns zero statements; exposed as a pull tour mode and described in demo/README.md.

Before / after

Before
  pull CLI ──> --help only; demo tour: lint ──> diff ──> apply

After
  docs/pull.md ──> README / docs index
  demo tour:  lint ──> diff ──> apply ──> pull ──> file count ──> diff --json == 0 statements