Group a session-metrics session's turns into higher-level SEMANTIC TASKS ("what was I actually trying to do") and render a Tasks companion page (*_tasks.html + *_tasks.md) with a worth-it / mixed / likely-waste verdict per task. Trigger when the user runs /task-breakdown, when session-metrics suggests a task breakdown after a JSON export, or when the user asks to "group my turns into tasks", "what tasks did this session cover", "which work was worth it vs wasted", or "break this session into tasks". Consumes the deterministic per-request breakdown (request_units) from a session-metrics JSON export — it never re-derives cost or token numbers. Args: $ARGUMENTS[0] = path to a session-metrics JSON export (optional; if omitted, generate one first).