fathom-verify

Run and interpret fathom's "done means proven" gate for any fathom-indexed repo. Use before checking off a docs/plans/*.md box, before a git push, when asked whether a plan's claims actually hold, or to see computed SDD progress (fathom plans status/next). Explains exit codes 0/1/2, --strict vs the default, --require-all, and the fail-open convention for an unreachable daemon or an empty index.