feat(ci): synth-per-variant build with github:* context in artifact (#91)

* 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>

* fix(ci): remove redundant git diff exclusion

The .gitignore already excludes /cdk/cdk-*.out/, so git add .
never stages synth output — the pathspec exclusion was belt-and-
suspenders with no effect.

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

* fix: remove explicit retention days for artifact uploads

Removed retention days for uploaded artifacts in build workflow.

* refactor(ci): use mise build synth output instead of redundant cdk synth step

Remove the separate `npx cdk synth` step — `mise run build` already
runs `cdk synth -q` via the //cdk:build task chain. Upload the
existing cdk/cdk.out/ directly as the variant artifact.

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

* feat(ci): pass computeVariant context via CDK_CONTEXT env var

CDK reads CDK_CONTEXT_<key> env vars as context values, so setting
CDK_CONTEXT_computeVariant=${{ matrix.variant }} flows the variant
through mise run build → cdk synth without modifying mise tasks.

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

* feat(ci): generate cdk.context.json with github:* tags before build

Replace CDK_CONTEXT env var with a cdk.context.json generation step
that populates all 13 github:* tag values plus computeVariant and
stackName. CDK reads cdk.context.json automatically during synth,
so tags are baked into templates and carried in the artifact for
deploy.yml to use without re-synthesis.

Event-specific resolution normalizes github.sha/ref/head-ref across
pull_request, merge_group, push, and workflow_dispatch triggers.
All GitHub context is passed through env: variables to prevent
script injection (CWE-78).

Refs: #73

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

* fix(ci): include cdk.context.json in CDK artifact upload

CDK does not copy cdk.context.json into cdk.out/ during synthesis.
Include it explicitly in the artifact so deploy.yml and release
assets carry provenance of what context produced the templates.

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

---------

Co-authored-by: scottschreckengaust <345885+scottschreckengaust@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: scottschreckengaust