feat(ci): add CDK synth-per-variant with artifact upload in build.yml

Add matrix strategy (agentcore variant) to build job. After the full
mise build, synthesize CDK output per variant and upload as immutable
artifact for downstream deploy workflow consumption.

Refs: #73

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scottschreckengaust