test(build-infra): add gitignore utils

[tylerbutler]

Description

Adds build-infrastructure gitignore utilities backed by tinyglobby and the ignore package, with direct tests for the intended gitignore semantics.

The tests cover ancestor and descendant .gitignore files, negation patterns, relative-path filtering, ordering expectations, and explicit cache clearing for long-running processes.

Reviewer Guidance

The review process is outlined on this wiki page.

Please focus review on the helper API, gitignore semantics, and cache behavior. The workspaceCompat migration is split into a follow-up PR.

[github-actions]

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Based on the diff (439 lines, 4 files), I've queued these reviewers:

• Correctness — logic errors, race conditions, lifecycle issues
• Security — vulnerabilities, secret exposure, injection
• API Compatibility — breaking changes, release tags, type design
• Performance — algorithmic regressions, memory leaks
• Testing — coverage gaps, hollow tests

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[Copilot]

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Files not reviewed (1)

• build-tools/pnpm-lock.yaml: Generated file

[github-actions]

Bundle size comparison

Base commit: 83f341cb4e76e4a7438a31a486fb317fb8a3e2e3
Head commit: f92b724c0ae973f69e73d5387b100fe6c81f4d49

Pending — Build - client packages is running. Results will appear here when the build completes.