TEMPORARY diagnostic: why the CI checkout reports itself shallow

[ChelseaKR]

TEMPORARY. DO NOT MERGE. This branch exists only to read one measurement off a runner and will be deleted.

tests/test_release_claims.py refuses to read the tag list when git rev-parse --is-shallow-repository says true. On every pull_request run since #68, it says true — even though actions/checkout is invoked with fetch-depth: 0 and the logged fetch carries no --depth. The same refspec reproduced locally on git 2.55.0 produces a non-shallow clone. This adds three diagnostic steps to find out where .git/shallow comes from.

Prepared with AI assistance; reviewed before submission.

[ChelseaKR]

Diagnostic complete; this branch has done its job and is being removed.

The finding: tools/a11y_browser was writing .git/shallow into this work tree. On a pull_request run Playwright's git-info plugin calls gitDiff, which begins git fetch origin <pr base sha> --depth=1 (node_modules/playwright/lib/runner/index.js), and the harness's working directory is inside the repository, so the fetch shallows the checkout. tests/test_release_claims.py then correctly refuses to read the tag list, and five tests fail on branches that changed nothing related. It presented as flake because whether it fired depended on pytest -n auto scheduling.

The fix is PR #74. Nothing here should be merged.

Prepared with AI assistance; reviewed before submission.