docs(plans): implementation plan for signatures, item 152

Eight tasks, TDD throughout, against
specs/2026-08-24-signatures-design.md.

Tasks 1 to 3 build the Signatures namespace: reading the directory, the
splice for both placements, and the match guard that keeps a "-- "
delimiter from authorising a deletion. Task 3 carries a mutation check on
that guard, since it is the one piece preventing data loss.

Task 4 adds the three config keys, task 5 the editor-bar switch and the
seeding, task 6 the From: follow that stops once the user chooses. Task 7
is documentation and the Italian translation; task 8 closes the backlog
item, and deliberately hands the work over for a hand test first rather
than marking it done on a green suite.

MessageBuilder is untouched by every task, which the plan states twice: the
signature lives in the composer's buffer and both MIME parts are already
derived from it.

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

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