Keep optional pnpm cache failures from blocking CI

[munirad7s]

Summary

• resolve the pnpm store through a guarded Hermit-aware helper with three attempts
• skip only optional cache restore/save when pnpm cannot yield a non-empty absolute path
• keep dependency installation and every Desktop Core/smoke/integration job mandatory
• add an always-run workflow contract covering success, retry, failure, empty output, and all cache consumers

Run 31636239395 showed Hermit's lazy pnpm download returning HTTP 503. The previous echo "STORE_PATH=$(pnpm store path --silent)" masked that command-substitution failure and passed an empty path to actions/cache.

Related issue

Closes #142

Testing

• bash -n scripts/resolve-pnpm-cache-path.sh
• bash -n scripts/test-ci-pnpm-cache-contract.sh
• bash scripts/test-ci-pnpm-cache-contract.sh
• parsed .github/workflows/ci.yml with PyYAML
• git diff --check
• just ci attempted twice; both local runs reached Clippy and were blocked only by an unchanged external sherpa-onnx archive download ending with Network Error: Unexpected EOF

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