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/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--