feat(signatures): splice a signature into a buffer

Both placements over one implementation. above_quote inserts before the
attribution rather than before the first quoted line: the attribution
introduces the quote and belongs with it, and a signature between the two
would read as part of the quoted message.

A buffer with no quote makes above_quote identical to end, so a new
message needs no branch of its own.

Part of item 152.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEcn3u19xPqv6ggD15PG4c

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