docs(specs): design signatures, item 152

One markdown file per signature under ~/.config/qtmaildir/signatures/,
spliced into the composer's buffer and switched from a control on the
editor bar.

MessageBuilder needs no change: it already builds text/plain from
markdownBody verbatim and text/html from MarkdownRenderer over the same
string, so one markdown signature in the buffer yields both forms
correctly. That is the "transparent to the user" requirement the note
asked for, and it is why a two-file text/HTML variant was dropped after
being chosen: it buys designed HTML signatures at the cost of the
signature no longer being visible while composing.

The switch stays stateless. seedBody() deliberately refuses to track
"my text" and "the quote" as separate pieces, and a toggle cannot duck
that question the way the quote did; it answers it by scanning for the
last "-- " block not followed by quoted lines, so nothing can desync
from the undo stack. That same scan is what lets signature_position
offer both end (the default) and above_quote over one implementation.

The per-account key does not reopen the note's constraint: an account
seeds the choice, the switch keeps every signature reachable, and the
automatic follow stops once the user picks one.

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

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