feat(ci): rename computeVariant to compute_type and apply as resource tag

[scottschreckengaust]

Summary

• Renames computeVariant → compute_type throughout the CI pipeline (build.yml matrix key, env var, cdk.context.json key) to align with the existing ComputeType union type in repo-config.ts
• CDK app now reads compute_type from context (default: agentcore) and applies it as a resource tag via Tags.of(stack).add('compute_type', computeType)
• Adds 2 new tests: default value assertion + explicit context value assertion

Closes remaining Phase 2 items in #73.

What changed

File	Change
.github/workflows/build.yml	matrix.variant → matrix.compute_type; env VARIANT → COMPUTE_TYPE; jq arg/key computeVariant → compute_type
cdk/src/main.ts	+3 lines: read compute_type context, apply as tag
cdk/test/stacks/github-tags.test.ts	Mirror production logic in helper + 2 new test cases

Test plan

• github-tags.test.ts — 5/5 pass (3 existing + 2 new)
• TypeScript compiles clean
• Pre-commit hooks pass (eslint, gitleaks, yaml check)
• CI build passes (will verify artifact name is cdk-agentcore-out)

Note: Pre-push security hook has a pre-existing failure (CVE-2026-27135 in libnghttp2 base image) unrelated to this change.

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