Add th seed deterministic demo data loader

[snissn]

Background

PoC demos need data quickly. Manually creating records via UI is slow and brittle.

Goal

Add first-class seed tooling for deterministic sample data loading.

Scope

• New command: th seed <schemaOrBuildDir> --file <seed.json>.
• Seed format supports per-collection create payloads.
• Optional idempotency mode (skip records already present by unique keys).
• Friendly summary output (created/skipped/failed counts).

Implementation Notes

• Reuse generated ABI/manifest surface to avoid duplicating contract assumptions.
• Keep seed format schema-driven and documented.

Acceptance Criteria

• Can seed canonical job-board from a JSON file in one command.
• Re-running with idempotency does not duplicate unique records.
• Errors identify collection + record + reason.
• Docs include seed file examples.