Add JSON schema validation for sop.json

[ldraney]

User Story

AS a dev-sop-engine user
I WANT sop.json validated before generation
SO THAT I catch config errors early with helpful messages

Current State

• No validation before generation
• Invalid config may cause silent failures or confusing errors

Expected Behavior

npx dev-sop-engine .
# Error: sop.json validation failed:
#   - rules.my-rule.events: must be an array
#   - agents.foo.prompt_file: file not found

Implementation Options

1. JSON Schema - Create sop.schema.json, validate with ajv
2. TypeScript - Runtime type checking with zod or similar
3. Both - Schema for editor autocomplete + runtime validation

Acceptance Criteria

• Invalid sop.json produces clear error message
• Missing required fields are caught
• Invalid field types are caught
• File path references are validated (files exist)