v0.7.0: Automatic handoff: continue on another agent when every account is limited

Summary

Failover already moves a session to another account of the same agent when it hits a usage limit, and csm handoff can move a task to another agent by hand. This PR connects the two: when a confirmed usage limit leaves no ready account of the running agent, csm saves the task context and hands it to the next agent you chose, on its own, in the same project.

What changes for users

$ csm auto automatic
$ csm auto handoff codex,gemini
Failover: automatic
Hand off to another agent: Codex → Gemini CLI when every account of the running agent is unavailable

• csm auto handoff <agent,...> | off sets the agents to hand off to, in order. It is off by default and only used when the failover policy is automatic.
• Same agent first. Another account of the running agent is still tried first and its native session resumed, exactly as before.
• Then the next agent. csm picks the first listed agent that is installed, has a ready signed-in account and can take an opening instruction, writes the task handoff (context, git state, MCP servers carried over), and starts it in the same project. Unusable agents are skipped, never started.
• Clear on screen: "work reached its usage limit and no other Claude Code account is ready. Automatic handoff: work (Claude Code) → codex-main (Codex)."
• Service and desktop: GET/PUT /v1/failover include handoff_to; an automatic handoff is recorded in the activity log as such.

Pull request: #7
Full changelog: v0.6.0...v0.7.0