PROVENANCE.md is out of sync with the exercises #31

scripts/check-provenance.sh (see the PR that adds it) checks that every exercise under CSwL/ is mentioned in PROVENANCE.md, and that every id in a CSwL id column is an existing exercise. On main (88f09f2) it reports:

Exercises with no entry in PROVENANCE.md

• implication-as-disj
• unfold-rw-conjunction

Ids in PROVENANCE.md that are no longer exercises

• add-zero-induction
• adjective-types
• and-intro
• apply-several-premises
• building-terms
• cases-on-or
• feature-value
• identity-implication
• p-implies-q-implies-p
• square-unfold
• twice
• unfold-conjunction
• preposition-phrase and complex-relative-clauses: these exist on feat/english-chapter and should resolve when feat(ModelChecking): write the chapter from CSwFP/6.1, 4.2, 6.2 and 6.3–6.4 #29 is merged.

For each missing exercise, check in CSwFP whether it has a counterpart, and add it to the right table (or to "Exercises original to CSwL"). For each stale id, find out whether the exercise was renamed or dropped, and update or remove the row. Also, is-weekend becomes prose in #30 and its row is removed there.

Done when scripts/check-provenance.sh exits with status 0.