P3.5: Docs — README + AGENTS.md + zellij smoke (multiplexer Phase 3)

[takemo101]

Phase 3, issue P3.5 — part of the multiplexer backend abstraction implementation plan.

Design: docs/designs/cuekit-multiplexer-backend-design.md

Prerequisites: P3.4 merged.

Outcome

Docs explain how to opt into zellij, what fallback looks like, and add an end-to-end smoke recipe for the zellij path.

Files

• Modify: README.md
• Modify: AGENTS.md
• Create: docs/guides/zellij-backend-smoke.md

Steps

• README: install section mentions zellij as an alternative; pin the minimum supported version (from P3.0 spike).
• AGENTS.md: add a pitfall row covering "zellij capture-pane formatting differs subtly from tmux".
• Smoke guide: replicate the gemini smoke pattern but exercise zellij spawn/attach/steer/kill.

Acceptance

Docs reviewable; smoke guide runnable.