docs(protocols): mechanical primary trigger for session-context writes

The "If this session crashed right now..." heuristic was the trigger. It pushes the decision onto the agent every turn, and the agent's bias is to defer when no obvious milestone just happened. Four recent sessions in the work project showed the same drift. The pattern: substantive work, no mid-session writes, a wrap-time reconstruction afterward.

The new primary trigger is mechanical. A turn that called any state-modifying tool (Edit, Write, Agent dispatch, MCP write, or Bash that mutates state) writes to the Session Log before the closing user-facing message. Pure-read turns (Read, Glob, Grep, read-only Bash) don't trigger. The existing high-loss bullets stay as elaboration. They aren't the trigger.

The 5-turn safety net remains. The judgment heuristic is gone. The primary trigger replaces it.

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