ci: add package install sanity check to build-and-test

[Hweinstock]

Description

Add a package install sanity check to the build-and-test workflow. After npm pack, the CI now installs the tarball globally and verifies the CLI boots and can scaffold a project. Catches packaging regressions (missing files, broken bin shims, miscategorized deps) before merge.

Related Issue

Closes #591

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe): CI improvement

Testing

The workflow itself is the test — it verifies the packaged tarball installs and runs correctly (agentcore --version, agentcore create).

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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