Add pause and restart controls for session agent processes

Feature summary

Add controls to pause a session and restart its underlying agent process while preserving the same session.

What problem are you trying to solve?

I want to temporarily stop a session's execution or restart its agent process without losing conversation history, workspace changes, or having to recreate the session. The operation should affect only the selected session, not require restarting the entire app.

Proposed solution

• Provide discoverable Pause session and Restart agent actions in the session UI.
• Pause execution without archiving or deleting the session, and clearly indicate the paused state.
• Restart the underlying agent process for the existing session, preserving its conversation history, workspace, uncommitted changes, and session configuration.
• Make lifecycle transitions and any failures visible so users can tell whether the session is paused, restarting, or ready.

Here, restart means restarting the agent process while keeping the session, not merely resuming a paused response or starting a new conversation.

Workflow impact

Gives users per-session execution control and a way to recover a stuck or unhealthy agent without disrupting other sessions or discarding existing work.

Additional context

Related: #2462 requests pausing work on a session. This request also covers explicitly restarting the agent process while preserving session continuity.

Start by locating the session UI and the agent-process lifecycle entry points; no files or tests are named in the issue. Done means per-session Pause session and Restart agent actions preserve conversation history, workspace changes, configuration, and session identity while showing paused, restarting, ready, and failure states without affecting other sessions.