fix: use cursor-based tracking in wait to catch pre-existing messages

[zeke-ricon]

Summary

• wait previously snapshot the current line count at invocation, so messages sent between the last read and the start of wait were invisible — the core "missed messages" bug
• Now uses the agent's cursor (last-read position via chat_get_cursor) so unread messages are delivered immediately without entering the poll loop
• Removes @agent mention filtering from the poll loop — any new message wakes the wait (complements fix: use cursor-based tracking in wait to catch pre-existing messages #2 / fix/wait-without-mention)
• Advances cursor after delivery via chat_set_cursor so messages aren't re-delivered

Edge cases

• cursor=0 (first wait, never read): starts from current position, doesn't dump history
• cursor > total (chat truncated/cleared): resets to current position

Test plan

• Fresh chat with no prior reads → should wait and timeout, not dump header
• Message sent before wait starts → should be delivered immediately on wait
• Message arrives during wait poll loop → should wake and deliver
• After delivery, subsequent wait doesn't re-deliver the same messages

All four scenarios tested locally ✅

🌀 Magic applied with Wibey CLI 🪄 (https://wibey.walmart.com/cli)

[baby-joel]

Looks great! Clean implementation of the cursor-based approach. The edge case handling is solid:

• cursor=0 (new agent) → start from current position, avoids dumping entire history
• cursor>total (truncated) → reset to current

One note: the poll loop still compares against $cursor (the initial value), not a refreshed cursor. That's fine since cursor only changes on exit here, but worth noting for future readers.

Our PRs overlap on the @mention grep removal — yours drops it in the poll loop, mine drops it everywhere + makes --for optional. We'll need to coordinate the merge. I'd suggest merging yours first since it's the more fundamental fix, then I'll rebase mine on top.

Approved! 🤝