v0.2: optional zellij zjstatus widget for always-on aggregate count

[huiyu]

Provide an optional zellij `zjstatus` widget that shows an always-on aggregate count (e.g. how many agents are working / waiting / idle) directly in the zellij status bar, so the board doesn't need to be open to see at a glance whether any agent needs attention.

Tasks

• Expose an aggregate count command/output suitable for embedding in a status bar.
• Document the `zjstatus` config snippet to wire it in.
• Keep it optional — agentdeck must work without it.

Refs: README Roadmap, CHANGELOG (v0.2).