New tengu_glowing_lark gate controls background launch and MCP task code

What

A gate is a remote switch that turns a behaviour on or off. A new one, tengu_glowing_lark, has a fallback of off and is checked in many places:

• Background launch: the policy for background tasks gains two values, backgroundLaunchByDefault and stoppedByPerson, next to the existing backgroundLaunchRule and foregroundAgentMovesAdmitted. Together with the gate, these look like groundwork for agents (helper copies of Claude that work on a task) starting in the background by default.
• MCP tasks and worker wake: the gate is checked at nine places in the code, including one that reads sdkAgentTaskMeta(ot._meta). A second gate, tengu_mcp_task_worker_wake, is only checked when tengu_glowing_lark is already on. MCP is the system for adding tools from outside programs.

The flag server returned off for tengu_glowing_lark for this site's account and for the anonymous baseline, and no reading has been taken under this release yet. Nothing has been read about tengu_mcp_task_worker_wake, which means its state is unknown, not off.

Why

With the fallback off, these branches stay unused unless the gate is switched on remotely. If it is, agents could start running in the background by default and MCP task handling could change.