fix(grading): stop autograder from silently crashing under bash 5

[lassebenni]

Summary

• ((var += expr)) reports a failing exit status whenever the resulting arithmetic value is 0 -- a well-known bash gotcha. Combined with set -euo pipefail, the grader silently dies mid-run the moment a student's running score hits exactly 0 at a checkpoint (e.g. a mostly-empty early submission).
• This only shows up on GitHub Actions' Ubuntu runners (bash 5+); it's invisible when testing locally on macOS (bash 3.2 ships without this strictness), which is how it went unnoticed.
• Also guards bare check_*() helper calls (which legitimately return 1 to signal warn/fail, not a script error) with || true so they don't trip set -e either.

Test plan

• bash -n .hyf/test.sh
• Reproduced against an empty scaffold repo with GNU bash 5: exit code 1 before the fix, exit code 0 after.
• Re-ran against a known-good local solution: score/pass output unchanged (regression-free).
• Confirm a real PR run in CI now completes the "Run tests" step for a low/zero-scoring submission.

🤖 Generated with Claude Code

[github-actions]

📝 HackYourFuture auto grade

Assignment Score: 39 / 100 ❌

Status: ❌ Not passed
Minimum score to pass: 60
🧪 The auto grade is experimental and still being improved

Test Details

=== Week 5 Autograder ===
  PASS: Level 1: required files (15/15 pts)
  FAIL: Dockerfile does not use python:3.11-slim base image
  FAIL: Dockerfile does not copy requirements before source code
  PASS: Dockerfile has a CMD instruction
  PASS: Level 2: Dockerfile (3/15 pts)
  PASS: tests/test_pipeline.py has 10 test functions (≥2 required)
  PASS: tests/test_pipeline.py has no NotImplementedError stubs remaining
  PASS: Level 3: unit tests (10/10 pts)
  FAIL: requirements.txt has no pinned packages (use package==version)
  PASS: Level 4: pinned dependencies (0/10 pts)
  PASS: ci.yml triggers on pull_request
  PASS: ci.yml triggers on push to main
  FAIL: ci.yml missing ruff check step
  FAIL: ci.yml missing ruff format step
  FAIL: ci.yml missing pytest step
  FAIL: ci.yml missing docker build step
  PASS: Level 5: CI workflow (8/20 pts)
  FAIL: pipeline.py does not read from os.environ or os.getenv
  FAIL: pipeline.py still contains NotImplementedError
  PASS: Level 6: env-var config (0/15 pts)
  FAIL: assets/acr_push_week5.png missing (Task 7 deliverable)
  PASS: Level 7: ACR screenshot (0/10 pts)
  PASS: AI_ASSIST.md has all three required sections
  FAIL: AI_ASSIST.md still contains TODO placeholders or is too short (681 chars)
  PASS: Level 8: AI report (3/5 pts)
  PASS: .gitignore correctly excludes __pycache__/, *.pyc, and .env

Score: 39 / 100  (passing: 60)  pass=false