fix(chat): GLM review batch — corruption, traversal, cursor truncation, double print (v4.6.78-80)

12 findings accepted, 3 refuted with evidence (see _docs/chat-review-glm-2026-10-23.md).

Data loss: a corrupt thread read as [] and the next append rewrote it with one message —
now 500 and the file is left alone. Cursor after a >50-message truncation pointed at the
thread tail, making the oldest permanently unfetchable — now the last message actually
sent, plus truncated:true. Two posts in the same millisecond shared a cursor — epoch is
now strictly increasing.

Security: chat.py show() had no id validation and joined argv onto CHAT_DIR ('../../x'
read any .json as darkbot) — THREAD_RE added. The auto-reply prompt now treats message
text as untrusted data and forbids acting on instructions found in it.

Ops: relay housekeeping swept neither .lock-* nor orphaned .tmp-*; a crashed listing
looked identical to an empty queue; grep AWAITING also matched message bodies; hermes-poll
printed every reply twice.

Verified: corrupt file -> 500 with the file untouched; traversal -> 'invalid thread id';
hermes-poll -> exactly one line per reply.

1 files changed, 10 insertions, 3 deletions

diff --git a/api/chat/poll.php b/api/chat/poll.php
index 601beca..b4aa970 100644
--- a/api/chat/poll.php
+++ b/api/chat/poll.php
@@ -14,14 +14,21 @@ $all = chat_read($thread);
 $messages = $since > 0
     ? array_values(array_filter($all, fn($m) => (int)($m['epoch'] ?? 0) > $since))
     : $all;
-$lastAll = $all ? end($all) : null;
+
+$window = array_slice($messages, -50);
+$truncated = count($messages) > count($window);
+// The cursor is the last message the client is actually being handed. Returning the
+// thread's real tail after a truncation would skip everything in between for good.
+$lastSent = $window ? $window[count($window) - 1] : null;
+$lastAll  = $all ? $all[count($all) - 1] : null;
 
 json_response([
     'success'  => true,
     'thread'   => $thread,
-    'messages' => array_slice($messages, -50),
+    'messages' => $window,
+    'truncated' => $truncated,
     // `awaiting` replaces `unread`: the server keeps no read state, so an unread
     // counter could never clear. Who spoke last is knowable.
     'awaiting' => (bool)($lastAll && ($lastAll['author'] ?? '') !== 'hermes'),
-    'cursor'   => $lastAll ? (int)($lastAll['epoch'] ?? 0) : 0,
+    'cursor'   => $lastSent ? (int)($lastSent['epoch'] ?? 0) : ($lastAll ? (int)($lastAll['epoch'] ?? 0) : 0),
 ]);