test: add tests for untested shared-ignore utilities

[google-labs-jules]

This PR adds unit tests for isNonProductDir, isNonProductPath, and walkFiles in cli/shared-ignore.mjs, which previously had zero direct coverage in any tests/ file. The new tests verify directory exclusion logic, deep path handling, and recursive file traversal mechanics (including conditional dotfile skipping). All existing and newly added tests pass successfully.

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PR created automatically by Jules for task 654466260464989592 started by @raccioly

[google-labs-jules]

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[github-actions]

🔎 Flagged for human review — this looks like a new, non-duplicate change from @jules. A maintainer should verify it against docguard guard + the test suite before merge.