feat(chat): token-scoped chat endpoints, contract, host CLI, old POST retired (v4.6.71 phase B)

- api/chat/{lib,post,poll}.php: thread = sha256(token)[0:16]; flock read-modify-write;
  tmp+rename; ms epoch cursor; per-thread + per-IP windows with retry_after_ms;
  401/413/507/405 carry a machine code, only rate_limited is retryable
- _docs/chat-contract.md: frozen shapes, incl. why the cursor is ms and not ts
- scripts/chat.py: list/show/reply, UTC, locked, atomic replace
- api/debug/agent-replies.php POST -> 410 Gone with pointers (one write path)

Measured: 401 no-token, 201+id, poll 1 msg, poll>since 1 new, data/chat 403,
4001 chars 413 (codepoints), 429 with Retry-After 576, round trip via chat.py.

1 files changed, 27 insertions, 0 deletions

diff --git a/api/chat/poll.php b/api/chat/poll.php
new file mode 100644
index 0000000..601beca
--- /dev/null
+++ b/api/chat/poll.php
@@ -0,0 +1,27 @@
+<?php
+// api/chat/poll.php — read back the caller's own thread. The thread comes from the
+// token, never from a query parameter: no id to guess, no IDOR, no listing.
+// Contract: _docs/chat-contract.md
+require_once __DIR__ . '/lib.php';
+// Note: poll is deliberately NOT rate limited. A 30 s poller is 120 GETs/hour;
+// sharing the write window would make the client throttle itself into silence.
+
+$token  = chat_token();
+$thread = chat_thread($token);
+$since  = isset($_GET['since']) ? (int)$_GET['since'] : 0;   // epoch seconds, numeric
+
+$all = chat_read($thread);
+$messages = $since > 0
+    ? array_values(array_filter($all, fn($m) => (int)($m['epoch'] ?? 0) > $since))
+    : $all;
+$lastAll = $all ? end($all) : null;
+
+json_response([
+    'success'  => true,
+    'thread'   => $thread,
+    'messages' => array_slice($messages, -50),
+    // `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,
+]);