Add Spec Kit documentation

Spec Kit — Civic Scam Shield

Adding the three core Spec Kit documents for the project:

• specs/001-civic-scam-shield/spec.md — Requirements & user stories
• specs/001-civic-scam-shield/plan.md — Technical blueprint & architecture
• specs/001-civic-scam-shield/tasks.md — Ordered implementation task list

These documents follow the Spec Kit workflow (specify → clarify → plan → tasks).