Optional: widen check_no_repo_root_syspath to src/ (0 hits today; needs own justification)

Goal

Extend workflow_lint.py::check_no_repo_root_syspath (tests/ + scripts/ as of #2183) to also scan src/, with its own justification and test coverage.

Why (and why optional)

src/ had 0 hits at both the #2181 enumeration and the #2183 widening — there is no live offender and no incident. #2183's task body (criterion 4) made this widening explicitly OPTIONAL and requiring separate justification; the #2183 plan (v2 §4d/§8 item 2) deliberately did not take it. Filed on_hold so the option is durable without entering the active proposed queue — revive via task.py set-status <N> proposed if a src/ offender ever appears or the justification is written.

Acceptance criteria (if taken)

1. Scan scope gains src/; CheckScope + docstring + argparse help updated.
2. A src/-scope firing case + green corrective-form case in tests/test_workflow_lint_no_repo_root_syspath.py.
3. Live tree green under the widened scope; no-flags bundle no new failures.

Provenance

Deferred from #2183 (plan v2 §8 item 2; task body acceptance criterion 4).