chore(hooks): log how many messages the sent carve-out matched

Item 164 diagnostics. A `notmuch tag` that matches nothing succeeds, so
the carve-out logged "applied over N folder(s)" whether it stripped
`inbox` from four messages or from none. A draft kept `inbox` on a pass
whose log claimed the carve-out had run, and that line could not tell
the two cases apart.

Count before tagging, since the tag is what makes the count zero, and
report it alongside the folder count. Nothing branches on the value: a
failed count yields `?` rather than failing the sync, because the tag
reports its own status separately.

This does not fix item 164, whose trigger is still unreproduced. It
makes the next occurrence self-explaining: `0 message(s)` means the
message was never in tag:new scope, and a non-zero count means the tag
ran over it and something re-added the tag afterwards.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UUQS6n3cmsFrsjCNmwNtf8

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