feat(cli): add dedicated CHAINLOOP_AI_AGENT_CONFIG material type with JSON schema validation

Replace the generic EVIDENCE material type with a dedicated CHAINLOOP_AI_AGENT_CONFIG
type for AI agent configuration data. This adds JSON schema validation when users
manually add materials via `chainloop attestation add`, matching the CHAINLOOP_PR_INFO
pattern.

Signed-off-by: Jose I. Paris <jiparis@chainloop.dev>

Author: jiparis