merge: the item 123 compose-and-send design into master

The seven brainstorm commits are documentation of decisions already taken:
the spec at docs/superpowers/specs/2026-08-20-compose-and-send-design.md,
the rewritten item 123 pointing at it, and the seven backlog items the
brainstorm opened (128 to 134). No implementation of 123 exists yet, so
nothing premature reaches master.

They are merged now because item 134 shipped to master as af902e0 while the
row defining it sat on the branch, which left master carrying code for an
item its own backlog had no record of. Item 132 is the same shape and is
next.

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