New stop-hook path for MCP end-of-turn requests behind tengu_glittery_mitten

What

A hook is a command you configure to run at certain points. A stop hook runs when Claude is about to finish its turn. MCP (Model Context Protocol) servers are add-ons that give Claude extra tools, and they can ask for a turn to end. This release adds a new code path for those requests.

• A new helper returns true only when the requested stop is mcp_meta_blockable, the current step is not the last one, and the gate tengu_glittery_mitten is on. A gate is a remote switch Anthropic uses to turn features on for some users.
• If looking up the gate fails with an error, the helper reports query_mcp_end_turn_stop_hooks / gate_threw and returns false.
• A companion helper logs [end-turn] bookkeeping threw and was skipped: when its bookkeeping fails, and carries on.
• query_mcp_end_turn_stop_hooks was also added to a list of names that looks like a list of behaviour flags.

Nothing has been read about tengu_glittery_mitten, so whether it is on is unknown.

Why

In practice this may let stop hooks act on end-of-turn requests from MCP servers. When the gate check fails, it is treated as off, and a failure in the extra bookkeeping does not end the turn early.