Add JSON schema validation for config.json

[mataeil]

There is no automated validation that config.json conforms to the expected schema. A user could have a typo in a field name, a missing required field, or an invalid type and would not know until /evolve fails mid-cycle.

Proposed solution:

• Create config.schema.json (JSON Schema draft 2020-12) from config.example.json
• Add a validation step to /ooda-config validate that checks against the schema
• Optionally add a scripts/validate.sh that checks all SKILL.md contracts and config schema

This is foundational for CI and for the skill contribution workflow.