feat: pause auto-continue on real user intervention

Detect a genuine human message that arrives after the plugin's most
recent continuation prompt and pause the goal (stop reason
"user intervention") so the plugin does not talk over the user; resume
via /goal resume. Plugin-generated continuation prompts (user-role
messages framed in <goal_continuation>) are ignored, and detection
requires turnCount > 0 plus a visible plugin continuation so the first
idle and scrolled-out sessions are never misread.

Adds isPluginContinuationMessage and userInterventionDetected helpers,
the idle-handler check, README/CHANGELOG, and unit + handler tests.
Implements items 5.2 and 5.3.