fix(hooks): keep inbox on mail that was sent AND received

notmuch deduplicates by Message-ID, so mail the user sends to another of
their own accounts is one message with two files: the sender's Sent copy
and the recipient's Inbox copy. The carve-out matched on a file's path
and tagged the message, so matching the Sent copy stripped inbox from
the copy that had genuinely arrived and the mail was missing from the
account that received it.

sent_only() keeps a message only when EVERY file is inside a sent
folder, which is what the docstring already claimed the predicate did.
It is a loop because no query can express it: measured against a
two-file message, `not path:` does not exclude it, and `count
--output=files` reports every file of every matching message rather
than the files that matched. Both read as if they worked, and are wrong
for the same reason, that a notmuch term is a predicate over a message
while this distinction is between its files.

The root comes from database.mail_root rather than database.path, since
this index is split and no message file is under the index directory. A
test fixture with the index outside the mail root covers it; the
ordinary layout cannot, because both keys return the same string there.

Both mutations fail: all->any loses inbox on the self-addressed message,
mail_root->path silently stops stripping anything. Verified read-only
against the live index, tagging nothing: of 807 messages matching a sent
path, 780 are still stripped and 27 are spared, every one of them two
files with one in another account's inbox. No arrival is affected.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HFuRPtzFrSxCQjFk6tq7gD

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