docs: specify compose and send, item 123

Brainstormed with the user. Design only, no code, which is what the
item's #plan-only tag asked for.

The decision that shaped everything: there is no MTA on the machine, so
"an external script on the same model as mailsync.sh" had no model to
copy. Send becomes a per-account send_command taking the message on
stdin, exactly as [sync] command already works, which keeps the
no-network-protocol rule intact without naming an MTA.

An account with no send_command is receive-only by construction, which
is how one of the five accounts is meant to work. Reply, reply-all and
forward are disabled on its mail behind a ribbon that says why.

The body is markdown parsed by cmark-gfm rather than a hand-written
parser for a limited set: the two share no code, so the small one is
deleted wholesale the moment the set widens.

Four new units, three of them widget-free and testable without a
painter. MessageSender is deliberately a separate unit rather than a
method on the composer, so a future outbox wraps the funnel instead of
reworking it.

Item 123's section is replaced by a pointer to the spec, per this
document's own rule for a fully specified item. The brainstorm opened
items 128 to 132, including a review of the every-action-has-a-shortcut
rule, which the user raised: six more actions takes it past the point
where a chord for everything is useful.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDq53rMd3AQp7QmcZzpuBM

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