History: enforce modes on existing paths, index turns, strip NUL

From review of the storage commit. makedirs and connect leave an existing
directory or file at whatever mode it had, so a history dir made earlier
with the default umask stayed 0755; both are now chmodded. A NUL in a
search ended the FTS string and raised instead of returning nothing. The
index on turns(conv, seq) serves the seq lookup per save, load_conv and the
cascade, which otherwise scan every turn. The self-check now also asserts
that an OR query matches nothing, which a non-raising but unquoted query
would not.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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