bashunit learn lesson gates are satisfied by their own hint comments

[Chemaclass]

Problem

Each lesson checks that the learner used the API it teaches:

if [ "$("$GREP" -c "assert_contains" "$test_file" || true)" -eq 0 ]; then
  echo "Your test should use assert_contains"

But the template that lesson generates contains the name in its own hint:

  # TODO: Check that message contains "bashunit"
  # Hint: assert_contains "substring" "$message"

so the gate passes before the learner writes anything. All 9 lessons with a grep gate are already satisfied by their own untouched template.

The consequence is not just a missing hint. A learner can complete a lesson without doing it:

function test_multiple_assertions() {
  local message="Hello, bashunit!"

  # TODO: Check that message contains "bashunit"
  # Hint: assert_contains "substring" "$message"

  assert_same 1 1
}

grep gate (assert_contains): 1 match — passes, on the comment
test run --fail-on-risky:    exit 0
=> "✓ Excellent! Lesson 2 completed!" without ever using assert_contains

The lesson teaches assert_contains, assert_matches and assert_not_empty; none is required in practice.

Fix

Count matches in code, not comments — the gate should look at what the learner wrote, not at the hint it printed. A shared helper in src/learn/session.sh used by all nine lessons, rather than nine copies of a grep -v '^[[:space:]]*#'.

Related

#1257 fixed the other half of this: an untouched template used to complete a lesson because a test with no assertions is risky and risky exits 0. That closed the "no work at all" path; this closes the "wrong work" path.

[Chemaclass]

Done in #1259 (merged).