Add pause and resume controls for an agent mid-task

[Cheggin]

Problem

Users need a way to pause an agent while it is in the middle of a task. Today the available controls do not clearly support pausing execution without ending the task, which makes it hard to intervene, inspect state, or temporarily stop work.

Scope

• Add a user-facing pause control for running agent sessions.
• Add a matching resume control so paused sessions can continue without creating a new task.
• Define what "paused" means for active browser automation, engine subprocesses, streaming output, queued tool calls, and session status.
• Make paused state visible in the session list, active task view, pill/log surfaces, and any relevant status indicators.
• Ensure cancel/stop behavior remains distinct from pause.

Acceptance criteria

• A running agent can be paused from the UI without marking the session failed or complete.
• A paused agent can be resumed and continue the same task/session.
• Paused sessions show an explicit paused status in the UI and persisted session state.
• Browser/tool activity does not continue unexpectedly while paused.
• Cancel/stop still terminates the task rather than merely pausing it.
• Tests cover pause/resume session state transitions and UI control visibility for running vs paused sessions.