P3.1: Add multiplexer config to .cuekit.yaml (multiplexer Phase 3)

[takemo101]

Phase 3, issue P3.1 — part of the multiplexer backend abstraction implementation plan.

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

Prerequisites: Phase 2 issues all merged.

Outcome

.cuekit.yaml accepts multiplexer: tmux | zellij and multiplexer_strict: boolean, parsed and validated. Default is tmux. No runtime effect yet.

Files

• Modify: packages/project-config/src/schema.ts
• Modify: docs/designs/cuekit-project-config-design.md (add new fields to the schema list)

Steps

• Add the two fields to the Zod schema.
• Default multiplexer to "tmux"; default multiplexer_strict to false.
• Add tests asserting parse/default/strict behaviour.

Acceptance

Project config tests pass with new fields parsing correctly.