feat: add --idle-timeout CLI flag for daemon auto-shutdown (#856)

* feat: add --idle-timeout CLI flag for daemon auto-shutdown

Add user-friendly --idle-timeout flag that converts time strings
to milliseconds. Supports formats like '10s', '3m', '1h', or raw ms.

This addresses a common need for ephemeral/CI environments where
daemon processes can be orphaned if not explicitly closed, leading
to resource consumption from zombie chrome-headless-shell processes.

Co-authored-by: Hermes (via claude-sonnet-4-20250520) <agent@hermes.ai>

* fix: address idle-timeout review feedback

* fix: normalize idle-timeout parsing

---------

Co-authored-by: Merlin <merlin@rbeckner.com>
Co-authored-by: Hermes (via claude-sonnet-4-20250520) <agent@hermes.ai>

cli/src/commands.rs

@@ -2181,6 +2181,7 @@ mod tests {
             screenshot_dir: None,
             screenshot_quality: None,
             screenshot_format: None,
+            idle_timeout: None,
         }
     }