hooks(fix): add doctest cleanup for tmux < 3.0 compatibility

why: On tmux 2.6-2.8, unset_hook without index doesn't clear all
array entries, causing state leakage between doctest examples
what:
- Add unset_hook cleanup after first set_hooks example
- Ensures clean state before clear_existing=True example

Squash target: 8a37592 (hooks refactor: rename set_hooks_bulk)

Author: tony

src/libtmux/hooks.py

@@ -449,6 +449,9 @@ def set_hooks(
         >>> sorted(hooks.keys())
         [0, 1]
 
+        >>> session.unset_hook('session-renamed')
+        Session($...)
+
         Set hooks from a list (sequential indices):
 
         >>> session.set_hooks('after-new-window', [