Create a task in a session's todo list

Mints a pending task in the agent's per-session todo store (B3b of the Agent Loop Kernel Relocation). Session scoping: task_create takes the driving agent session from its invocation, so the model never supplies it; a route caller may pass session_id explicitly, else the shared default bucket is used, and only the conversation's owner may use its list. The route mirrors task_create: that tool's Deny, Ask and firewall judge the call (403 tool_refused, or consent_required to a caller that declares the REST approval channel), and the call is recorded as a task_create call.