P3.2: ZellijBackend basic implementation (multiplexer Phase 3)

[takemo101]

Phase 3, issue P3.2 — part of the multiplexer backend abstraction implementation plan.

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

Prerequisites: P3.0 spike closed (cite its findings); P3.1 merged.

Outcome

A working ZellijBackend implements MultiplexerBackend that mirrors tmux's per-task-session model: 1 task → 1 cuekit-task-<id> zellij session. No team awareness in this issue — that's Phase 4.

Files

• Create: packages/adapters/src/zellij-backend.ts
• Create: packages/adapters/__tests__/zellij-backend.test.ts

Steps

• Use the spike output (P3.0) for exact CLI forms.
• Implement spawnPane, isAlive, sendKeys, capturePane, killPane, attachCommand. All session-name patterns: cuekit-task-<task_id>.
• Use a FakeZellijRunner in tests to assert exact argv shapes.
• Real-zellij integration test gated on a hasZellij() helper (analogous to hasTmux()).

Acceptance

Unit tests pin argv shapes; integration test (when zellij present) submits a one-shot task and round-trips spawn → capture → kill.