runVersionMatchGate and runCanvasStructureGate read locator content with git show/cat-file. For a tag-name locator, `git fetch origin <tag>` only updates FETCH_HEAD and never creates refs/tags/<tag>, so `git show <tag>:...` died with 'invalid object name' and produced a false infra_error.
Read the primary locator via HEAD (already checked out during clone) and non-primary locators via FETCH_HEAD after fetching, instead of the bare locator. This handles SHAs, short tag names, and fully-qualified tag refs uniformly without classifying the locator.
Adds regression coverage for the tag-locator path in both gates.
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>