feat(library): seed 50 curated activity templates

[tomiloki]

Context

Phase 2 del re-design. Poblar `activity_templates` con la biblioteca curada que el agente usará como menú interno.

Spec & Plan

• Spec: `docs/superpowers/specs/2026-05-08-habitquest-reimagination-design.md` (sección 4.4)
• Plan: `docs/superpowers/plans/2026-05-08-habitquest-reimagination.md` (Phase 2)

Tasks

• Crear `lib/ai/seed-templates.ts` con 50 templates tipados (10 movement / 8 focus / 8 rest / 6 nutrition / 6 digital_balance / 6 social / 6 creativity)
• Crear `scripts/build-seed-migration.ts` que genera el SQL bulk insert desde el TS
• Aplicar migration 004 (`seed_library`)

Acceptance

```sql
SELECT category, count(*) FROM activity_templates GROUP BY category;
```
Devuelve exactamente: movement=10, focus=8, rest=8, nutrition=6, digital_balance=6, social=6, creativity=6.

Depends on

Issue de Phase 1 cerrada (la migration 003 agrega las columnas que el seed usa).