Address Kyle's review comments: working-directory and jq // empty comment

[Copilot]

Two unresolved review comments from Kyle on PR #427, incorporating all underlying changes from that branch.

Changes

• clang-tidy-check.yaml: Replace explicit cd "$GITHUB_WORKSPACE/${BUILD_PATH}" with the working-directory: step option, consistent with the pattern in python-check.yaml. Removes the now-redundant BUILD_PATH env var.

  # Before
  env:
    BUILD_PATH: ${{ needs.setup.outputs.build_path }}
  run: |
    cd "$GITHUB_WORKSPACE/${BUILD_PATH}"
    ...
  
  # After
  working-directory: ${{ needs.setup.outputs.build_path }}
  run: |
    ...

• dependabot-auto-merge.yaml: Add a comment on the "Get PR details" step explaining why // empty is used in jq expressions — it produces an empty string rather than the literal "null" when a key is absent or null, so the downstream validation checks work correctly.

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