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/fetch_test.go

@@ -0,0 +1,50 @@
+package email
+
+import (
+	"net"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestFetchSinceRun(t *testing.T) {
+	mk := func(subject string) string {
+		return "Subject: " + subject + "\r\nContent-Type: text/plain; charset=utf-8\r\n\r\nbody\r\n"
+	}
+	f := &fakeIMAP{
+		uids: []uint32{1, 2, 3},
+		msgs: map[uint32]string{1: mk("one"), 2: mk("two"), 3: mk("three")},
+	}
+	fs := FetchSince{
+		Addr: "mail.example:993", User: "kami", Mailbox: "INBOX",
+		Timeout: 5 * time.Second,
+		Since:   time.Date(2026, 7, 30, 0, 0, 0, 0, time.UTC),
+		Max:     2,
+		Skip:    func(uid uint32) bool { return uid == 3 },
+		dial: func(addr string, timeout time.Duration) (*Conn, error) {
+			cli, srv := net.Pipe()
+			go f.serve(t, srv)
+			return NewConn(cli, timeout)
+		},
+	}
+	msgs, err := fs.Run("secret")
+	if err != nil {
+		t.Fatalf("run: %v", err)
+	}
+	// Newest first, the already-seen UID skipped, Max respected.
+	if len(msgs) != 2 {
+		t.Fatalf("got %d messages, want 2: %+v", len(msgs), msgs)
+	}
+	if msgs[0].Subject != "two" || msgs[1].Subject != "one" {
+		t.Errorf("subjects = %q,%q, want two,one (newest first)", msgs[0].Subject, msgs[1].Subject)
+	}
+	if strings.Contains(strings.Join(f.cmds, " "), "UID FETCH 3") {
+		t.Error("a skipped UID must not be fetched again")
+	}
+}
+
+func TestFetchSinceRequiresConfig(t *testing.T) {
+	if _, err := (FetchSince{}).Run("secret"); err == nil {
+		t.Fatal("an unconfigured mailbox must not be read")
+	}
+}