[codex] fix Ruff CI scope

[melody-ling-L]

What changed

• Configure astral-sh/ruff-action with src: ./src instead of embedding src/ inside args.
• Pin Ruff to 0.15.18, matching the version used by the failing run.
• Remove three genuinely unused imports from src/data_prep.py and src/eval_runner_v2.py.

Root cause

ruff-action appends its src input to the Ruff command. Because the workflow left src at its default repository root while also passing src/ inside args, CI executed effectively:

ruff check src/ ... <repository-root>

That unintentionally linted notebooks, generated reports, and scripts in addition to src/, producing a persistent red badge.

Impact

CI now enforces the repository's intended policy: E/F/W checks on production source code under src/. The source tree itself is clean; this does not suppress real source errors or change benchmark results.

Validation

• Reproduced the failing command from GitHub Actions logs.
• Verified the official Ruff Action src input behavior.
• Ran Ruff 0.15.18 locally with the exact intended command: ruff check --select E,F,W --ignore E501 ./src.
• Result: All checks passed!
• python3 -m compileall -q src passed.
• Workflow YAML parsing and git diff --check passed.