cron_lesson_consolidate: an uncreatable log dir silently skips the whole nightly pass

Goal

Make scripts/cron_lesson_consolidate.sh fail loud when it cannot write its own daily log file, instead of silently skipping the entire lesson-consolidation pass and exiting 0.

The bug

scripts/cron_lesson_consolidate.sh runs the consolidator inside a brace group redirected to its daily log:

mkdir -p "$LOG_DIR"
...
{
    echo "=== ... start ==="
    cd "$PROJECT_DIR" || exit 1
    uv run python scripts/consolidate_lessons.py --apply --window-days 7
    rc=$?
    echo "=== ... exit=$rc ==="
} >> "$LOG_FILE" 2>&1
...
exit 0

There is no set -e, and mkdir -p is unchecked. If $LOG_DIR cannot be created — ENOSPC on / (chronically near-full and shared across ~15 concurrent sessions), a permissions change, a bad EPS_LESSON_CONSOLIDATE_LOG_DIR override — then:

1. mkdir -p fails, and nothing notices.
2. The brace group's >> "$LOG_FILE" redirect fails, so the entire group never executes — the consolidator is never invoked at all.
3. rc is never assigned.
4. The wrapper reaches exit 0.

Net effect: the nightly lesson-consolidation pass silently does not run, and the only place that would have recorded the fact is the log file that could not be created. Every marker in the window keeps re-accumulating unprocessed. There is no cron email on this VM (no MTA; the crontab line redirects 2>&1), so nothing else surfaces it either.

Reproduced directly:

EPS_LESSON_CONSOLIDATE_LOG_DIR=/proc/definitely_not_writable \
EPS_LESSON_CONSOLIDATE_BIN=/bin/true \
bash scripts/cron_lesson_consolidate.sh
# → wrapper exits 0; consolidator never ran; nothing written anywhere

This is the same failure CLASS as #2190 (a real condition that produces no notification), on the same file, but a DISTINCT condition: #2190 surfaces a consolidate_lessons.py exit-3 budget refusal — a pass that RAN and refused; this is a pass that never ran at all.

How it surfaced

Found by the #2190 code-review round-1 pass as a Minor. #2190's diff moved the rc read outside the brace group, which under set -u turned this path into rc: unbound variable / exit 1 — an accidental, undiagnostic behaviour change from a change meant only to ADD an alert path. #2190 restored the pre-diff behaviour with ${rc:-0} and deliberately did NOT widen its scope to fix the underlying silence, filing it here instead.

Suggested direction (not prescriptive — the planner decides)

The wrapper already has the right precedent a few lines above: the uv-on-PATH check fails loud to stderr and exit 1s, with a comment explaining that the trailing exit 0 would otherwise hide it (task #580). A mkdir -p failure deserves the same treatment — check it, write a FATAL line to stderr naming the path, and exit non-zero — plus, given #2190 has now wired a Telegram channel into this wrapper, consider routing this condition through it too, since stderr alone reaches nobody under cron.

Whatever shape is chosen must keep #2190's invariants intact: routine rc=0 passes stay silent, the consolidator's exit semantics are untouched, and the tests/test_cron_lesson_consolidate.py pins (T1-T9) stay green.

Acceptance criteria

• An uncreatable $LOG_DIR produces a loud, diagnostic failure naming the path — not a silent exit 0.
• The failure is pinned by a test in tests/test_cron_lesson_consolidate.py driving the wrapper with an unwritable log dir.
• All existing T1-T9 pins stay green; rc=0 passes stay silent.
• No change to scripts/consolidate_lessons.py, the crontab schedule, or --window-days.

Provenance

Surfaced by the #2190 code-reviewer (round 1) as a non-blocking Minor on commit d43fc3687c; #2190 applied the one-token ${rc:-0} guard (commit 912f4f3a95) to avoid shipping an unintended behaviour change, and filed this task for the underlying condition rather than widening its own approved scope.