Add th export-demo portable PoC bundle command

[snissn]

Background

PoC handoff is messy when consumers need to reconstruct the local workflow manually.

Goal

Provide one-command export of a portable demo bundle for reproducible PoC sharing.

Scope

• New command: th export-demo <schema> --out <dir>.
• Bundle includes schema, generated app, run instructions, and expected local prerequisites.
• Include manifest/build metadata and version stamps.

Implementation Notes

• Optimize for local reproducibility, not production deployment.
• Keep output deterministic and self-describing.

Acceptance Criteria

• A teammate can run the exported bundle following included instructions.
• Bundle clearly states toolchain requirements and launch steps.
• Export command has integration test coverage.