ai(rules[AGENTS]): document tmux_stdout_len and tmux_stderr_len keys

why: These keys are used in common.py and neo.py but were only
mentioned in passing, not in the key table.
what:
- Add tmux_stdout_len and tmux_stderr_len to Heavy/optional keys table

Author: tony

AGENTS.md

@@ -315,6 +315,8 @@ Pass structured data on every log call where useful for filtering, searching, or
 |-----|------|---------|
 | `tmux_stdout` | `list[str]` | tmux stdout lines (truncate or cap; `%(tmux_stdout)s` produces repr) |
 | `tmux_stderr` | `list[str]` | tmux stderr lines (same caveats) |
+| `tmux_stdout_len` | `int` | number of stdout lines |
+| `tmux_stderr_len` | `int` | number of stderr lines |
 
 Treat established keys as compatibility-sensitive — downstream users may build dashboards and alerts on them. Change deliberately.