docs: close out compose and send, and drop the v1/v2 split

Task 13 of item 123, the documentation that was left when the code was
finished.

The README claimed "sending is not implemented. Compose, reply, forward and
send are planned for v2 and need a companion send script that does not exist
yet", which had been false for three days. It also documented none of
`trash`, `send_command` or the `[compose]` section: `trash` has been effectively
mandatory since item 103 and its absence breaks Delete, so a user reading only
the README could not configure a working install. Both are item 122, which
closes with this.

Every default in the new [compose] block was read out of config.h rather than
from the prose around it, which was worth doing: send_html defaults to TRUE
and the first draft of this documented it as false, and the key is
autosave_interval_ms rather than the autosave_delay_ms that reads more
naturally.

The v1/v2 language is retired rather than updated. The project follows semver
on its user-visible surface and those labels described a scope split that
stopped being true when compose shipped. CLAUDE.md says so explicitly rather
than deleting the sentence, since the older spec and plan documents still use
the phrase and are historical records: read it there as "before compose".

CLAUDE.md's architecture diagram listed none of the compose units. It does
now, and it distinguishes the classes from the namespaces, because most of
them are namespaces of free functions over values: the markdown, the MIME
assembly and the account-picking are all testable without a widget. There is
no FormatToolbar class, which the first draft of this diagram invented, and
that is the exact mistake the paragraph under it already warns about for
QueryBar and AttachmentBar.

Also records item 148 from the notes: Ctrl+W does not close the composer.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q2koFevoSxTLhfexJTZWQd

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