atperson: split main.cpp into scoped CLI command atoms

atperson 96a3c66 closes the last remaining monolith from issue #42's checklist: src/app/main.cpp no longer combines environment helpers, path resolution, usage text, and seventeen command bodies in one translation unit.

What moved

Atom	Owns
`cli/config.{hpp,cpp}`	Environment variables, durable-path resolution, bounded integer parsing
`cli/usage.{hpp,cpp}`	The command and environment reference text
`cli/ledger_maintenance.{hpp,cpp}`	`rebuild`, `compact`, `withdraw`
`cli/graph_inspection.{hpp,cpp}`	`assoc`, `candidates`, `familiarity`, `recall`
`cli/ingest.{hpp,cpp}`	`ingest`, `ingest-file`
`cli/cursor.{hpp,cpp}`	`cursor status`, `cursor reset`
`cli/sync.{hpp,cpp}`	`sync` — client construction, per-page budget refresh, cursor-rejection recovery

Every body is moved byte-faithfully: behaviour, ordering, and output text are unchanged. main.cpp keeps only process bootstrap (home-directory bootstrap notice), snapshot load-or-create, resource probing, and one dispatch stanza per command — 231 lines, down from 500.

Conventions

Atoms follow the established pattern: explicit parameters with the same identifiers the moved statements used, plus caller-owned callbacks (print_stats, usage) so text rendering stays with main. Network-only atoms (cursor, sync) construct their own Wolfram-backed AtprotoClient from the documented environment, keeping credentials out of the dispatch layer.

Both ATPERSON_BUILD_NETWORK=OFF and ON configurations build and pass all 32 tests; CLI smoke-tested end to end (ingest, stats, assoc, candidates, familiarity, recall, withdraw, rebuild, compact).