docs(export): define the versioned trace JSON contract

[stackloomdev]

Context

CauseScope can export a redacted JSON trace, but contributors and downstream tools do not yet have a public reference for the payload or its compatibility boundary.

Scope

• Add a documentation page for the current exported trace structure using only synthetic data.
• Identify the format/version field and define what can change in a patch, minor, or major CauseScope release.
• Document redaction, response-size, timeline, and total-recording bounds that affect exports.
• Link the reference from configuration, privacy, and the documentation sidebar.
• Add a regression check that keeps the documented synthetic example aligned with the exported TypeScript shape.

Acceptance

• A reader can determine whether two exports are structurally compatible without inspecting implementation files.
• The example contains no real application data or credentials.
• pnpm check passes.