docs: explain deterministic data diff sampling

[kvlonge]

Why

Users need a precise contract for limiting high-volume data diffs and interpreting sampled results without confusing them with exhaustive equality.

Changes

• Explain deterministic bounded-union key sampling and configuration precedence.
• Document explicit exhaustive overrides, exact cursor coverage, and key-integrity behavior.
• Distinguish comparison sample size from diagnostic example limits.
• Document JSON output and invocation safety limits.
• Correct path-default and direct-connection descriptions while updating the affected pages.

Verification

• Regenerated the packaged SQLBuild documentation skill from this worktree
• git diff --check
• Private public-repository scanner and configured pre-push hook