feat(scope): opt-in corpus scoping — scope.json + prune + scoped sync (#7)

A scope.json in the data directory now names what to KEEP: sources,
apple-docc frameworks, and whether fonts/SF Symbols stay. `apple-docs
prune` trims an existing corpus to it without re-crawling (documents,
FTS rows, vectors, raw payloads, relationships, and on-disk files all
go; VACUUM reclaims the space; --dry-run previews). `sync` reads the
same file, filtering adapters and apple-docc roots so refreshes never
grow past the scope, and skips the Xcode enrichment merge that would
flood excluded pages back in.

No scope.json → byte-identical full-coverage behavior everywhere; the
framework narrowing applies only to the apple-docc adapter so other
sources' root namespaces are never filtered by it.