ai(rules[AGENTS]): add tmux_option_key to logging key table

why: The options module uses tmux_option_key in extra dicts but it
was missing from the documented core keys table.
what:
- Add tmux_option_key row to the Core keys table in AGENTS.md

Author: tony

AGENTS.md

@@ -307,6 +307,7 @@ Pass structured data on every log call where useful for filtering, searching, or
 | `tmux_session` | `str` | session name |
 | `tmux_window` | `str` | window name or index |
 | `tmux_pane` | `str` | pane identifier |
+| `tmux_option_key` | `str` | tmux option name |
 
 **Heavy/optional keys** (DEBUG only, potentially large):