CodeRabbit flagged docstring coverage. Added docs where a caller would
actually need them — that `seedDevDatabase` is destructive and not
incremental, that it validates before opening anything so a rejected run
leaves the target intact, that the caller still owns the attachment
files, and that `DevSeedError.hint` is the actionable half the CLI prints
separately.

Left the self-describing internals (`placeholders`, `hasTable`,
`columnsOf`, `stateDbPath`) alone: the surrounding scripts/ code documents
about 2% of its declarations, so padding to an 80% tool default would
read as noise rather than convention.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>