Read a mailbox read-only, in a client small enough to audit (#246)

internal/email is the reading half of the email reader: a ~200-line IMAP
client (LOGIN, EXAMINE, UID SEARCH SINCE, UID FETCH BODY.PEEK, LOGOUT), a
MIME-to-plaintext converter, and a header-only junk filter.

Two protocol choices are the design, not shortcuts. EXAMINE instead of
SELECT means the session is read-only at the protocol level, so no command
in it can flip a flag or expunge anything by mistake. BODY.PEEK instead of
BODY means reading a message does not mark it \Seen — Maven reads his mail
and leaves no trace of having done so, and the unread state in his own
client stays his.

Hand-rolled rather than go-imap because this is the one path that holds his
mailbox credential and reads his private mail: five commands with no
dependencies is auditable in a sitting. No IDLE and no cleartext/STARTTLS
either — an option to send his password over a plain socket is an option to
get it wrong once.

Junk is decided by headers alone, before any model is involved:
List-Unsubscribe/List-Id, Precedence: bulk, Auto-Submitted, the spam
headers, and Gmail's own category labels. Sender lists and subject keywords
are deliberately absent — they age badly and they would put his contacts in
a config file. A junk verdict only means "do not spend the model on this";
nothing is deleted and no server flag is touched.

Nothing here logs a body, a subject or an address, the junk reason names a
header rather than content, and an undecodable charset degrades to
headers-only instead of feeding the model mojibake. Verified against
recorded .eml fixtures and an in-process fake IMAP server.

internal/email/testdata/cp1251.eml

@@ -0,0 +1,7 @@
+From: legacy@example.org
+To: kami@example.org
+Subject: Legacy
+Date: Fri, 01 Aug 2026 05:00:00 +0400
+Content-Type: text/plain; charset="windows-1251"
+
+Ï

internal/email/testdata/html_only.eml

@@ -0,0 +1,13 @@
+From: billing@isp.example
+To: kami@example.org
+Subject: =?utf-8?B?0KHRh9GR0YIg0LfQsCDQuNC90YLQtdGA0L3QtdGC?=
+Date: Fri, 01 Aug 2026 08:00:00 +0400
+MIME-Version: 1.0
+Content-Type: multipart/alternative; boundary="B1"
+
+--B1
+Content-Type: text/html; charset="utf-8"
+Content-Transfer-Encoding: base64
+
+PGh0bWw+PGhlYWQ+PHN0eWxlPnB7Y29sb3I6cmVkfTwvc3R5bGU+PC9oZWFkPjxib2R5PjxwPtCh0YfRkdGCINC30LAg0LjQvdGC0LXRgNC90LXRgjogNzAwJm5ic3A74oK9PC9wPjxwPtCe0L/Qu9Cw0YLQuNGC0Ywg0LTQviA1INCw0LLQs9GD0YHRgtCwLjwvcD48c2NyaXB0PngoKTwvc2NyaXB0PjwvYm9keT48L2h0bWw+
+--B1--

internal/email/testdata/mixed_attachment.eml

@@ -0,0 +1,26 @@
+From: hr@work.example
+To: kami@example.org
+Subject: Contract
+Date: Fri, 01 Aug 2026 07:00:00 +0400
+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary="M1"
+
+--M1
+Content-Type: multipart/alternative; boundary="A1"
+
+--A1
+Content-Type: text/plain; charset="utf-8"
+
+Sign the contract before Monday.
+--A1
+Content-Type: text/html; charset="utf-8"
+
+<p>Sign the contract before Monday.</p>
+--A1--
+--M1
+Content-Type: application/pdf; name="contract.pdf"
+Content-Disposition: attachment; filename="contract.pdf"
+Content-Transfer-Encoding: base64
+
+JVBERi0xLjQgbm90IHJlYWxseSBhIHBkZg==
+--M1--

internal/email/testdata/newsletter.eml

@@ -0,0 +1,9 @@
+From: news@shop.example
+To: kami@example.org
+Subject: =?utf-8?B?0KHQutC40LTQutC4INGC0L7Qu9GM0LrQviDRgdC10LPQvtC00L3Rjw==?=
+Date: Fri, 01 Aug 2026 06:00:00 +0400
+List-Unsubscribe: <mailto:unsub@shop.example>
+Precedence: bulk
+Content-Type: text/plain; charset="utf-8"
+
+Sale!

internal/email/testdata/plain_ru.eml

@@ -0,0 +1,14 @@
+From: =?utf-8?B?0JDQvdGC0L7QvQ==?= <anton@example.org>
+To: kami@example.org
+Subject: =?utf-8?B?0J3Rg9C20L3QviDQt9Cw0LrRgNGL0YLRjCDQt9Cw0LTQsNGH0YM=?=
+Date: Fri, 01 Aug 2026 09:12:00 +0400
+Content-Type: text/plain; charset="utf-8"
+Content-Transfer-Encoding: quoted-printable
+Message-ID: <plain-ru@example.org>
+
+=D0=9F=D1=80=D0=B8=D0=B2=D0=B5=D1=82! =D0=9D=D0=B0=D0=B4=D0=BE =D0=BE=D1=82=
+=D0=BF=D1=80=D0=B0=D0=B2=D0=B8=D1=82=D1=8C =D0=B0=D0=BA=D1=82 =D0=B4=D0=BE =
+=D0=BF=D1=8F=D1=82=D0=BD=D0=B8=D1=86=D1=8B.
+
+--
+Anton