workflow-fix: verify_task_body warn on body-linked figures untracked at branch HEAD

Overview / Motivation

Auto-filed by the workflow-fix-on-bug protocol from a prose follow-up surfaced by the interpretation-critic on task #841 (interp-critique v5, 2026-07-04).

Goal

Add a warn-level check to scripts/verify_task_body.py that every body-linked figures/issue_<N>/... path also resolves in git ls-tree at the issue-branch HEAD (not only at the pinned SHA), so silent figure-tracking loss cannot hide behind immutable SHA-pinned raw.githubusercontent URLs.

Workflow gap

• Bug observed: three body-linked parent figure stems (hero_matched_information, exploratory_atlas_meancentered, exploratory_transport_fidelity_cosine under figures/issue_841/) were tracked at the pinned SHA 4824a567aa but UNTRACKED at branch HEAD — local copies byte-identical, so every SHA-pinned body URL still rendered and nothing surfaced the loss.
• Why it is a workflow gap: verify_task_body.py checks that figure URLs resolve (HTTP) and are SHA-pinned, but a pinned URL is immutable — it keeps resolving forever even after the file falls out of tracking on the live branch, so a later merge/rebase can silently drop the canonical figure files from main with zero verifier signal.
• Confidence (emitter): high (the incident is concrete and reproducible; the check is mechanizable).

Proposed change (candidate diff sketch — refine in planning)

• In verify_task_body.py, for each body-linked raw.githubusercontent figure URL matching figures/issue_/...:
• path = extract repo-relative path from URL
• present_at_head = git ls-tree -r --name-only <issue-branch-or-HEAD> -- <path> non-empty
• (resolve the branch as issue- when it exists, else HEAD of the checkout the verifier runs in)
• absent -> WARN (not FAIL): "body-linked figure tracked at pinned SHA but missing at branch HEAD — re-commit it" WARN-level only (the pinned URL still renders; this is drift hygiene, not a broken body).

Scope / surfaces

• Primary target: scripts/verify_task_body.py
• Also touch its tests (tests/test_verify_task_body.py) with a fixture for the tracked-at-SHA-untracked-at-HEAD case.

Constraints / invariants

• Workflow-surface only. WARN, never FAIL (grandfathered bodies with fully-merged branches must not regress).
• scripts/workflow_lint.py --check-asks passes; ruff passes; existing verify_task_body tests stay green.

Provenance

• workflow_fix_target: scripts/verify_task_body.py
• fingerprint: aee0b9bd1097

Surfaced prose (verbatim, from the interpretation-critic's report on #841): "Suggest scripts/verify_task_body.py gain a warn-level check that every body-linked figures/issue_<N>/... path also resolves in git ls-tree at the issue-branch HEAD, so silent figure-tracking loss can't hide behind immutable pinned URLs; also re-commit the three files on issue-841." (The re-commit half was executed directly by the orchestrator: commit c7eba35ada on issue-841.)