docs: include agent_failure_modes in a hidden toctree

Clears the pre-existing Sphinx warning "agent_failure_modes.md: document isn't
included in any toctree" (the doc existed but was unlinked on main; unrelated to
the PyAutoBuild→PyAutoHands rename). A :hidden: toctree includes it in the doc
tree without changing the visible sidebar nav, so the zero-warning docs gate
passes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: claude

docs/index.md

@@ -69,3 +69,9 @@ adoption/guide
 adoption/config_surfaces
 example
 ```
+
+```{toctree}
+:hidden:
+
+agent_failure_modes
+```