Diagnose setup by what is certain, and deliver errors the way the client asked

Review of the branch found the persona diagnosis was mostly truncation
wearing a name tag. The <.> marker sits at the start of the system
message and the persona marker after the card, so a card Janitor cut
short arrives with the prompt present and the persona gone — and was
told its persona was wrong, with truncation as an afterthought. The
original message was only ever about context size, which is the best
evidence there is about what actually happens.

diagnoseSetup() now says only what the markers make certain. A missing
<.> is a missing prompt, since truncation cannot remove the first three
characters. A persona marker that is present but not "." is a wrong
description, quoted back so there is nothing to guess. A persona marker
that is gone, or cut off mid-tag, leads with truncation and mentions the
persona second.

The error path decided "streaming?" from the Accept header while the
success path used body.stream, so a client that sent stream:true without
that header got JSON back for every error and never displayed it. The
flag is hoisted above the try; the header is only a fallback for when
the body could not be read.

Token search is restricted to after the greeting, in order. A card built
from a sucker-proxied chat could carry an expired token in its text, and
a lookup on it would fail forever. Conversation order also means the
token a chat was started with always wins over one pasted later. The
system fallback now takes a role-less first message only; a request with
roles but no system turn is reported as such rather than misdiagnosed as
a missing prompt. The echo feature reads the parser's lastUser instead of
its own case-sensitive lookup. "Nothing was saved" is dropped from the
unknown-error message, since it is not guaranteed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NX2SsmUqQftWTZAY4BDuFa

tests/messages.test.ts

@@ -62,13 +62,26 @@ test("the token is still found once later replies stop carrying it", () => {
   assert.match(found!, /\[sucker:conv=abc123\]/);
 });
 
-test("a token the user just pasted outranks an older one", () => {
+test("the token a chat was started with outranks one pasted later, so a chat cannot flip between cards", () => {
   const candidates = tokenCandidates([
     ...withProbe,
     { role: "assistant", content: `created (v1). ${TOKEN}` },
     { role: "user", content: "{{user}}: [sucker:conv=zzz999]" },
   ]);
-  assert.match(candidates[0], /zzz999/);
+  const first = candidates.find((c) => c.includes("sucker:conv="));
+  assert.match(first!, /abc123/);
+});
+
+test("a token in the card or the greeting is never a candidate", () => {
+  const stale = "[sucker:conv=stale00]";
+  const candidates = tokenCandidates([
+    { role: "system", content: `${CARD} made with sucker: ${stale}` },
+    { role: "user", content: "." },
+    { role: "assistant", content: `${GREETING} ${stale}` },
+    { role: "user", content: NAME },
+  ]);
+  assert.ok(!candidates.some((c) => c.includes(stale)), "an expired token baked into a card would make it unextractable forever");
+  assert.deepEqual(candidates, [NAME]);
 });
 
 test("roles are matched whatever their casing", () => {
@@ -106,6 +119,17 @@ test("messages with no usable content are skipped, not treated as slots", () =>
   assert.equal(slots.nameSlot, NAME);
 });
 
+test("a request with roles but no system turn has no card, rather than a misdiagnosed one", () => {
+  const slots = selectMessageSlots([
+    { role: "assistant", content: GREETING },
+    { role: "user", content: NAME },
+  ]);
+  // Taking the greeting as the card would produce a "custom prompt not set"
+  // message for a request the user could never have fixed.
+  assert.equal(slots.system, undefined);
+  assert.equal(slots.greeting, GREETING);
+});
+
 test("a card with no role on the system turn still resolves", () => {
   const slots = selectMessageSlots([
     { content: CARD },