Show Operator widgets in Matrix rain

[edwin-zvs]

Summary

• Render the collapsed Operator/orchestrator session's normal sticky widgets as a transient viewport inside the Matrix-rain panel.
• Auto-show the most recently updated Operator widget, keep normal widget lifecycle semantics, and return to rain when the viewport hides.
• Add Matrix-rain title-bar widget count/status plus previous/next/toggle affordances for browsing the Operator widget stack.
• Document the design in specs/0019-operator-widgets-use-matrix-rain-viewport.md.

Testing

• cargo check -p agentd-cli
• cargo test -p agentd-cli matrix_rain
• cargo build

Notes

This PR only touches the TUI client (crates/cli) and a TUI spec, so the relevant binary is agent.