Ship dev PoC schema presets (job-board, marketplace, directory, social)

[snissn]

Background

New users need high-quality starting points beyond a blank schema.

Goal

Ship curated schema presets and generation shortcuts for common app patterns.

Scope

• Add presets: job-board, marketplace, directory, social.
• th init --preset <name> support.
• Presets include schema + sensible UI defaults and sample seed file (if available).
• Docs page comparing preset tradeoffs.

Implementation Notes

• Keep presets deterministic and versioned.
• Validate presets through existing lint/test pipeline.

Acceptance Criteria

• Users can scaffold each preset in one command.
• Preset schemas validate and generate successfully.
• Canonical tests include at least one non-job-board preset smoke path.