P1.3: Adapter factories accept MultiplexerBackend (multiplexer Phase 1)

[takemo101]

Phase 1, issue P1.3 — part of the multiplexer backend abstraction implementation plan.

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

Prerequisites: P1.2 merged.

Outcome

createClaudeCodeAdapter, createGeminiAdapter, createOpencodeAdapter, createJcodeAdapter, createPiAdapter, and the shared createPaneAdapter all type their panes parameter as MultiplexerBackend. Production wiring continues to pass a TmuxBackend.

Files

• Modify: packages/adapters/src/{claude-code,gemini,opencode,jcode,pi}-adapter.ts
• Modify: packages/adapters/src/pane-adapter.ts
• Modify: packages/adapters/src/build-registry.ts

Steps

• Replace PaneBackend parameter type with MultiplexerBackend in all factory signatures.
• build-registry.ts: confirm the constructed instance is typed as MultiplexerBackend.
• No new tests; existing tests should pass.

Acceptance

Typecheck passes. Existing FakeTmuxRunner-based adapter tests pass unchanged. Refactor commit is behaviour-preserving.