Log a practice session

Append one day to data/progress.txt — the hand-written practice log that site/build-review-plan.js compiles into the review plan's data on every build. It is the only copy. Nothing else in the repo records that a problem was practised, so a line this skill writes badly is a day that quietly leaves the schedule.

Invocation: /lc-log <what you practised> — e.g. /lc-log 23 ok, 24 again!!, 25. With no argument, ask what was practised today; do not invent a session.

Prime directives