[FEATURE]: Native infinite agent mode (auto-continue until done, with limits)

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Long migrations stall because the agent stops and the user must manually send "continue". I maintain an external supervisor (dorlanpabon/opencode-infinite-agent) that keeps sending continuations until a sentinel ([TASK_COMPLETE]) or all session todos are done, bounded by max iterations / timeouts. As an external project it is tedious to integrate fully — this should be native:

• Infinite mode per session, toggled in the composer next to agent/model/effort (Mode: Complete | Infinite), plus an infinite config section (maxIterations, maxHours, sentinel, todoDetection).
• When active, the runner auto-continues an idle-but-unfinished drain (sentinel missing and todos open) until done or limits hit. Never auto-continue while a permission/question is pending; existing interrupt stays as the stop button.

Related: #19384, #20874.