workflow-fix: guard grep-pattern literal false positive

Overview / Motivation

Auto-filed by the workflow-fix-on-bug protocol from the 2026-07-18 /daily transcript problem sweep (c5-P12). Route-2 filing.

Goal

Extend the repo-root branch guard so a read-only grep/egrep/fgrep/rg clause whose quoted PATTERN argument contains a destructive-op literal is waived even when a trailing file path follows the pattern (the destructive text lives inside the search pattern, not an executable git verb).

Workflow gap

• Bug observed: at 17:00Z on 2026-07-18 (AFTER #1501's heredoc-FP fix merged) the guard BLOCKED a read-only grep -n -m 3 -A 20 "test_worktree_revert_shapes_block[git reset --hard]" /tmp/step9c-pytest-issue-1513.log because the quoted pattern carried the literal git reset --hard.
• Why it is a workflow gap: the guard already waives grep/egrep/rg pattern arguments, but only when the quoted payload is the clause's FINAL token; a trailing log-file path after the pattern defeats that narrowing, so a read-only grep with a destructive-op literal in its pattern and a file argument fails CLOSED. This is a new false-positive class distinct from #1501's heredoc payloads.
• Confidence (emitter): medium
• verified-at-filing: grep -cn 'FINAL token' scripts/guard_repo_root_branch.sh → 2 hits (the grep/rg pattern-arg waiver is scoped to the quoted-payload-is-final-token shape; a trailing file path is not the final token so the waiver does not fire); git log --oneline --since='3 days ago' -- scripts/guard_repo_root_branch.sh → 4 commits (d534bdf299 #1501 heredoc, c55596834c, 124e9bec60, a87f5898f6), none widening the grep-pattern-arg waiver to allow a trailing file path (2026-07-19)

Proposed change (candidate diff sketch — refine in planning)

# in the grep/egrep/fgrep/rg read-only waiver span:
# CURRENT: waive only when the quoted pattern is the clause's FINAL token
# NEW: waive a grep-family clause whose destructive literal is confined to a
#      quoted PATTERN argument even when non-option file-path tokens trail it
#      (still refuse an unquoted destructive git verb anywhere in the clause).

Scope / surfaces

• Primary target: scripts/guard_repo_root_branch.sh
• Grep the guard's pattern-arg waiver span; extend the fail-closed quoted-separator handling to permit trailing file-path tokens after a grep-family quoted pattern. Add a self-test / test fixture for the grep-with-trailing-file shape (fails pre-fix, passes post-fix).

Constraints / invariants

• Workflow-surface only. Must NOT loosen the guard for an unquoted destructive git reset --hard / git checkout verb; the widening is scoped strictly to grep/egrep/fgrep/rg pattern-argument literals.
• scripts/workflow_lint.py --check-asks passes; ruff on touched files passes; the guard's existing self-test matrix stays green.
• This session runs under EPM_WORKFLOW_FIX_SESSION=1 and carries a workflow_fix_target: Provenance line — recursion guard applies.

Provenance

• workflow_fix_target: scripts/guard_repo_root_branch.sh
• fingerprint: 81ba2f948208

Surfaced problem (c5-P12, 2026-07-18 17:00Z): a read-only grep over a step9c pytest log was guard-blocked because its quoted search pattern contained git reset --hard and a trailing log-file path followed the pattern.