Allow creating a new sidebar session programmatically (for context handoff between agents)

Requesting an API/tool (or CLI equivalent) to create a new session that appears in the desktop app's left-hand session panel, so an agent can spawn a fresh session as part of a handoff workflow — e.g. summarizing context and starting a new agent session with it, without the user manually clicking "+ new session". Codex supports this kind of programmatic session creation; Claude Code currently does not expose it to the agent itself.